Electrode structure for lithium secondary battery and secondary battery having such electrode structure

ABSTRACT

An electrode structure for a lithium secondary battery including: a main active material layer including a metal powder selected from silicon, tin and an alloy thereof that can store and discharge lithium by electrochemical reaction, and a binder of an organic polymer; and a current collector. The main active material layer includes a powder of a support material for supporting the electron conduction of the main active material layer in addition to the metal powder and the powder of the support material are particles having a spherical, pseudo-spherical or pillar shape with an average particle size of 0.3 to 1.35 times the thickness of the main active material layer. The support material is one or more selected from graphite, oxides of transition metals and metals that do not electrochemically form alloy with lithium. Organic polymer compounded with a conductive polymer is used for the binder.

This application is a divisional of Application Ser. No. 11/296,460,filed Dec. 8, 2005 now U.S. Pat. No. 7,615,314, the contents of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electrode material for a lithium secondarybattery (lithium rechargeable battery), which is formed from the powderof particles containing a metal such as silicon or tin that forms analloy with lithium by electrochemical reaction, to an electrodestructure including such an electrode material and also to a secondarybattery having such an electrode structure.

2. Related Background Art

The fear for warming the surface of the earth due to the so-calledhothouse effect because of the increasing ratio of CO₂ gas contained inthe atmosphere has been pointed out in recent years. Thermoelectricpower plants convert the thermal energy obtained by burning fossil fuelinto electric energy but it has been made difficult to build newthermoelectric power plants because they give off CO₂ gas by a largequantity. To get rid of this problem, leveling the load ofthermoelectric power plants, so-called load leveling, by storing thesurplus electric power that is produced during the night in secondarybatteries installed in ordinary houses for the daytime when electricpower is consumed at an enormous rate has been proposed in order toeffectively use the electric power generated by the generators of powerplants including thermoelectric power plants.

Besides, development of secondary batteries with a high energy densityis expected for electric automobile applications that are characterizedby not emitting substances that contaminate the atmosphere such as CO₂,NO_(x) and hydrocarbons. Additionally, development of small, lightweightand high performance secondary batteries is an urgent issue for powersource applications in the field of portable appliances such asnotebook-sized personal computers, video cameras, digital cameras,batteryular phones and PDAs (personal digital assistants).

So-called rocking chair type “lithium ion batteries” have been developedand made commercially available for family use as compact, lightweightand high performance secondary batteries. A rocking chair type lithiumion battery is prepared by using a lithium intercalation compound thatde-intercalates lithium ions by a reaction that takes place at the timeof charging as a positive electrode material and a carbon material,which is typically graphite that can intercalate lithium between twolayers of the planes of the carbon hexagons formed by carbon atoms as anegative electrode material.

However, the above “lithium ion battery” cannot realize a secondarybattery with a high energy density that is comparable to a lithiumprimary battery using metal lithium as a negative electrode materialbecause maximally only ⅙ of a lithium atom per carbon atom can betheoretically intercalated into the negative electrode formed from acarbon material. If lithium is tried to be intercalated at a rate higherthan the theoretically possible rate into the carbon-made negativeelectrode of the “lithium ion battery” at the time of charging or if the“lithium ion battery” is tried to be charged with electricity in a highelectric current density condition, metal lithium grows on the surfaceof the carbon-made negative electrode as dendrite (showing a form oframification) to eventually end up with internal short-circuitingbetween the negative electrode and the positive electrode as a result ofrepetition of a charging/discharging cycle. Thus, “lithium ionbatteries” having a satisfactory cycle life have not been provided ifthe graphite electrode is made to show a capacity that exceeds thetheoretically possible capacity level.

Meanwhile, high capacity lithium secondary batteries having a negativeelectrode made of metal lithium have been attracting attention assecondary batteries showing a high energy density but have not achievedany commercial success yet because the cycle life of such batteries isshort. The reasons for such a very short cycle life are believed toinclude that metal lithium reacts with impurities such as moisture inthe electrolyte solution and the organic solvent to form an insulatingfilm and that the surface of the metal lithium foil is not plane but hasspots where an intense electric field is found to grow dendrite of metallithium that by turn give rise to internal short-circuiting between thenegative electrode and the positive electrode.

There have been proposed techniques to use an alloy of lithium andaluminum for the negative electrode in order to suppress the progress ofthe reaction between metal lithium and moisture in the electrolytesolution and the organic solvent, which is one of the problems ofsecondary batteries having a negative electrode made of metal lithium.However, the lithium alloy is very hard and cannot be wound in a spiralform and hence it is not possible to prepare spiral type cylindricalbatteries. Additionally, the cycle life is not extended as expected andthe energy density comparable to a battery having a negative electrodeof metal lithium has not been achieved. Thus, such techniques have notbeen commercially successful so far for these reasons.

In order to solve the above-described problems, the inventors of thepresent invention have proposed negative electrodes made of silicon, tinand the like for lithium secondary batteries in U.S. Pat. Nos.6,051,340, 5,795,679 and 6,432,585 and Japanese Patent ApplicationLaid-Open Publication Nos. H11-283627, 2000-311681 and InternationalPublication No, WO00/17949, More specifically, U.S. Pat. No. 6,051,340proposes a lithium secondary battery that uses a negative electrodewhere an electrode layer is formed by using metals of silicon and tinthat form an alloy with lithium and metals of nickel and copper that donot form any alloy with lithium on a current collector of a metalmaterial that does not form any alloy with lithium either. U.S. Pat. No.5,795,679 proposes a lithium secondary battery that uses a negativeelectrode formed form a powdery alloy of elements such as nickel orcopper and elements such as tin. U.S. Pat. No. 6,432,585 proposes alithium secondary alloy that uses a negative electrode of which theelectrode material layer contains particles of silicon and tin having anaverage particle diameter of 0.5 to 60 μm at an amount of 35 wt % ormore to show a void ratio from 0.10 to 0.86 and a density from 1.00 to6.56 g/cm³. Japanese Patent Application Laid-Open No. H11-283627proposes a lithium secondary battery that uses a negative electrodecontaining silicon and tin having an amorphous phase. Japanese PatentApplication Laid-Open No. 2000-311681 proposes a lithium secondarybattery that uses a negative electrode made of particles of amorphoustin and an alloy of a transitional metal to show a non-stoichiometriccomposition. International Publication No. WO00/17949 also proposes alithium secondary battery that uses a negative electrode made ofparticles of amorphous silicon and an alloy of a transitional metal toshow a non-stoichiometric composition.

Japanese Patent Application Laid-Open No. 2000-215887 proposes a lithiumsecondary battery that has a high capacity and shows a highcharging-discharging efficiency achieved by forming a carbon layer onthe surfaces of metal or half-metal particles, particularly siliconparticles, that can form an alloy with lithium by a chemical depositionprocess involving thermal decomposition of benzene to raise the thermalconductivity and suppress the expansion of the volume when forming thelithium alloy for the purpose of preventing destruction of theelectrode. However, the above cited proposal for a lithium secondarybattery using such an alloy type negative electrode is accompanied bydrawbacks including that the surfaces of silicon particles cannot becoated uniformly by a chemical deposition process involving thermaldecomposition and that the thermal decomposition temperature is high andapt to give rise to oxidation of silicon particles. Therefore, theproblem that the internal resistance increases as thecharging/discharging cycle is repeated, and consequently the rate oftaking out electricity gradually falls is not dissolved sufficiently incomparison with a lithium secondary battery having a graphite electrode.

The negative electrode of a lithium secondary battery that is formed byusing powder of a metal selected from silicon, tin and an alloy thereofthat can store and discharge lithium by electrochemical reaction and abinder expands as the battery is charged, and the negative electrodecontracts as the battery is discharged. Then, as theexpansion/contraction cycle is repeated, the contact of the metalparticles is decreased to allow metal particles to fall and the currentcollector to peel off from the electrode layer probably because thereaction of forming an alloy of metal particles and lithium unevenlytakes place during charging. Although attempts have been made to improvethe uneven reaction and make it more uniform by mixing the material ofthe negative electrode with carbon particles such as graphite particles,the electrochemical reaction relating to the charging/dischargingoperations of the lithium secondary battery does not take placeuniformly in the electrode layer because of the difference of electricstorage capacity and volume expansion between metal particles and carbonparticles in the case of increasing the amount of lithium that is storedin the negative electrode during charging (metal particles are units ofmetal powder and carbon particles are units of carbon powder).

Thus, there is a demand for development of negative electrodes that canprovide a long service lifetime in order to dissolve the above-describedproblems.

SUMMARY OF THE INVENTION

In view of the above-described points, the object of the presentinvention is to provide an electrode structure to be used for a lithiumsecondary battery that shows a small capacity reduction even when thecharging/discharging cycle is repeated and has a high capacity and ahigh energy density, and a lithium secondary battery including such anelectrode structure.

In the first aspect of the present invention, there is provided anelectrode structure for a lithium secondary battery, including: a mainactive material layer of a metal powder of a material selected from thegroup consisting of silicon, tin and an alloy thereof that can store anddischarge lithium by electrochemical reaction, and a binder of anorganic polymer; and a current collector;

wherein the main active material layer further includes at least apowder of a support material for supporting the electron conduction ofthe main active material in addition to the metal powder;

the powder of the support material are particles having a spherical,pseudo-spherical or pillar shape with an average particle size of 0.3 to1.35 times the average thickness of the main active material layer; and

the support material includes one or more materials selected from thegroup consisting of graphite, an oxide of a transitional metal selectedfrom TiO₂, MoO₃ and WO₃, a metal electrochemically not forming an alloywith Li which is selected from Cu, Ni, Co, Ti, Fe, Cr, Mo, W, Pd, Pt andAu and an alloy thereof.

In the second aspect of the present invention, there is also provided anelectrode structure for a lithium secondary battery, including: a mainactive material layer including a metal powder of a material selectedfrom the group consisting of silicon, tin and an alloy thereof that canstore and discharge lithium by electrochemical reaction, and a binder ofan organic polymer; and a current collector;

wherein (a) the main active material layer further includes at least apowder of a support material for supporting the electron conduction ofthe main active material layer in addition to the metal powder; thepowder of the support material are particles having a spherical,pseudo-spherical or pillar shape with an average particle size of 0.3 to1.35 times the thickness of the main active material layer; the supportmaterial includes one or more materials selected from the groupconsisting of graphite, an oxide of a transitional metals selected fromTiO₂, MoO₃ and WO₃, a metal electrochemically not forming an alloy withLi which is selected from Cu, Ni, Co, Ti, Fe, Cr, Mo, W, Pd, Pt and Auand an alloy thereof; and

wherein (b) an electron-conductive buffer layer is arranged between thecurrent collector and the main active material layer of the electrodestructure; the buffer layer includes at least a binder of an organicpolymer, and particles of one or more materials selected from the groupconsisting of a conductive polymer, graphite, a metal electrochemicallynot forming an alloy with Li which is selected from Cu, Ni, Co, Ti, Fe,Cr, Mo, W, Pd, Pt and Au and an alloy thereof, and an oxides of atransitional metal selected from TiO₂, MoO₃ and WO₃; and the averageparticle size of the particles is 0.5 μm to 10 μm.

In the third aspect of the present invention, there is also provided anelectrode structure for a lithium secondary battery, including: a mainactive material layer including a metal powder of a material selectedfrom the group consisting of silicon, tin and an alloy thereof that canstore and discharge lithium by electrochemical reaction, and a binder ofan organic polymer; and a current collector;

wherein (a) the main active material layer further includes at least apowder of a support material for supporting the electron conduction ofthe main active material layer in addition to the metal powder; thepowder of the support material are particles having a spherical,pseudo-spherical or pillar shape with an average particle size of 0.3 to1.35 times the thickness of the main active material layer; the supportmaterial includes one or more materials selected from the groupconsisting of graphite, an oxide of a transitional metal selected fromTiO₂, MoO₃ and WO₃, a metal electrochemically not forming an alloy withLi which is selected from Cu, Ni, Co, Ti, Fe, Cr, Mo, W, Pd, Pt and Auand alloy thereof;

wherein (c) a surface coat layer is arranged on the surface of the mainactive material layer; the surface coat layer has electron conductivityand ion transmissibility or ionic conductivity; the surface coat layerincludes at least a binder of an organic polymer, and particles of oneor more materials selected from the group consisting of a conductivepolymer, amorphous graphite, graphite, a metal electrochemically notforming an alloy with Li which is selected from Cu, Ni, Co, Ti, Fe, Cr,Mo, W, Pd, Pt and Au and an alloy thereof, and an oxide of atransitional metal selected from TiO₂, MoO₃ and WO₃; and the averageparticle size of the particles is 0.5 μm to 10 μm.

In the fourth aspect of the present invention, there is also provided anelectrode structure for a lithium secondary battery, including: a mainactive material layer including a metal powder of a material selectedfrom the group consisting of silicon, tin and an alloy thereof that canstore and discharge lithium by electrochemical reaction, and a binder ofan organic polymer; and a current collector;

wherein (a) the main active material layer further includes at least apowder of a support material for supporting the electron conduction ofthe main active material layer in addition to the metal powder; thepowder of the support material are particles having a spherical,pseudo-spherical or pillar shape with an average particle size of 0.3 to1.35 times the thickness of the main active material layer; the supportmaterial includes one or more materials selected from the groupconsisting of graphite, an oxide of a transitional metal selected fromTiO₂, MoO₃ and WO₃, a metal electrochemically not forming an alloy withLi which is selected from Cu, Ni, Co, Ti, Fe, Cr, Mo, W, Pd, Pt and Auand an alloy thereof;

wherein (b) an electron-conductive buffer layer is arranged between thecurrent collector and the main active material layer of the electrodestructure; the buffer layer includes at least a binder of an organicpolymer, and particles of one or more materials selected from the groupconsisting of a conductive polymer, graphite, a metal electrochemicallynot forming an alloy with Li which is selected from Cu, Ni, Co, Ti, Fe,Cr, Mo, W, Pd, Pt and Au and an alloy thereof, and an oxide of atransitional metal selected from TiO₂, MoO₃ and WO₃; the averageparticle size of the particles is 0.5 μm to 10 μm; and

wherein (c) a surface coat layer is arranged on the surface of the mainactive material layer; the surface coat layer has electron conductivityand ion transmissibility or ionic conductivity; the surface coat layerincludes at least a binder of an organic polymer, and particles of oneor more materials selected from the group consisting of a conductivepolymer, amorphous graphite, graphite, a metal electrochemically notforming an alloy with Li which is selected from Cu, Ni, Co, Ti, Fe, Cr,Mo, W, Pd, Pt and Au and an alloy thereof, and an oxide of atransitional metal selected from TiO₂, MoO₃ and WO₃; and the averageparticle size of the particles is 0.5 μm to 10 μm.

In the fifth aspect of the present invention, there is provided asecondary battery including a negative electrode formed by using theabove-described electrode structure, a lithium ion conductor, and apositive electrode, wherein an oxidation reaction of lithium and areduction reaction of lithium ions are utilized.

The present invention has been achieved under the above-describedcircumstances, and the first feature of the present invention is that,in an electrode structure to be used for a lithium secondary battery,the electrode structure includes: a main active material layer includinga metal powder of a material selected from the group consisting ofsilicon, tin and an alloy thereof that can store and discharge lithiumby electrochemical reaction, and a binder of an organic polymer; and acurrent collector; wherein the main active material layer furtherincludes at least a powder of a support material for supporting theelectron conduction of the main active material layer in addition to themetal powder; and the powder of the support material has a spherical,pseudo-spherical or pillar shape with an average particle size of 0.3 to1.35 times the thickness of the main active material layer. Preferably,the support material includes one or more materials selected from thegroup consisting of graphite, an oxide of a transitional metal (which isselected from TiO₂, MoO₃ and WO₃), a metal electrochemically not formingan alloy with Li (which is selected from Cu, Ni, Co, Ti, Fe, Cr, Mo, W,Pd, Pt and Au and an alloy thereof). More preferably, the supportmaterial is graphite. Preferably, the average particle size of theprimary particles that form the particles of the metal of the mainactive material layer selected from silicon, tin and alloys of theelements is 0.02 μm to 5 μm. Preferably, the ten point average height Rzshowing the surface roughness of the current collector is 0.7 μm to 3μm.

The second feature of the present invention is that anelectron-conductive buffer layer that expands only little during thecharging operation is arranged between the current collector and themain active material layer of the electrode structure; and the bufferlayer includes at least a binder of an organic polymer, and particles ofone or more materials selected from the group consisting of a conductivepolymer, graphite, a metal electrochemically not forming an alloy withLi (which is selected from the group consisting of Cu, Ni, Co, Ti, Fe,Cr, Mo, W, Pd, Pt and Au and an alloy thereof), and an oxide of atransitional metal (which is selected from TiO₂, MoO₃ and WO₃), and theaverage particle size of the particles is 0.5 μm to 10 μm.

The third feature of the present invention is that a surface coat layeris arranged on the surface of the main active material layer in order toalleviate the concentration of electric field that occurs during thecharging/discharging operation; the surface coat layer has electronconductivity and ion transmissibility or ionic conductivity; the surfacecoat layer includes at least a binder of an organic polymer, andparticles of one or more materials selected from the group consisting ofa conductive polymer, graphite, a metal electrochemically not forming analloy with Li (which is selected from the group consisting of Cu, Ni,Co, Ti, Fe, Cr, Mo, W, Pd, Pt and Au and an alloy thereof) and an oxideof a transitional metal (which is selected from TiO₂, MoO₃ and WO3); andthe average particle size of the particles is preferably 0.5 μm to 10μm.

Another feature of the present invention is that the same binder is usedfor the main active material layer, the buffer layer and the surfacecoat layer.

The fourth feature of the present invention is that a conductive organicpolymer is dispersed in the binder.

Still another feature of the present invention is that there is provideda secondary battery including: a negative electrode formed by using theabove-described electrode structure, an electrolyte, and a positiveelectrode, wherein an oxidation reaction of lithium and a reductionreaction of lithium ions are utilized, and that the powder of thesupport material in the main active material layer of the negativeelectrode shows an expansion coefficient of 1.5 times or less based onpowder before charging in the direction toward the oppositely disposedpositive electrode.

The fifth feature of the present invention is that, in an electrodestructure to be used for a lithium secondary battery, the electrodestructure includes: a main active material layer including a metalpowder of a material selected from the group consisting of silicon, tinand an alloy thereof that can store and discharge lithium byelectrochemical reaction, and hard carbon (non-graphitic carbonmaterial) powder or graphite carbon power, and a binder of an organicpolymer; and a current collector, wherein the metal powder and thecarbon powder are compounded by a material having a “link” function ofcarrying out the chemical bond or electron conduction between the metalpowder and the carbon powder (hereinafter, referred to as “linkmaterial”).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-sectional view of an embodiment of theelectrode structure according to the present invention, and FIG. 1B is aschematic cross-sectional view showing the state of the presentelectrode structure during the charging operation;

FIG. 2 is a schematic cross-sectional view of another embodiment of theelectrode structure according to the present invention;

FIG. 3 is a schematic cross-sectional view of still another embodimentof the electrode structure according to the present invention;

FIGS. 4A and 4B are schematic cross-sectional views of still anotherembodiment of the electrode structure according to the presentinvention;

FIG. 5A is a schematic cross-sectional view of still another embodimentof the present electrode structure where flat graphite particles arecompounded, and FIG. 5B is a schematic cross-sectional view showing thestate of the present electrode structure during the charging operation;

FIG. 6 is a schematic cross-sectional view of an embodiment of asecondary battery (lithium secondary battery) according to the presentinvention;

FIG. 7 is a schematic cross-sectional view of a single layered flat type(coin type) battery;

FIG. 8 is a schematic cross-sectional view of a spiral type cylindricalbattery; and

FIGS. 9A and 9B are schematic images of metal particles and a carbonparticle connected to each other by a link material in an electrodestructure according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventors of the present invention have achieved the presentinvention as a result of elaborate studies on alloy-based negativeelectrodes for lithium secondary batteries. In lithium secondarybatteries where the negative electrode is an electrode prepared byforming an active material layer of powder of metal silicon or tin or analloy thereof and a binder on a current collector of a metal foil, theinternal resistance of the battery increases as a result of repetitivecharging/discharging cycles to consequently degrade the performance ofthe battery. The inventors of the present invention observed andanalyzed the active material layer of the negative electrode to obtain apresumption that the increase of the internal resistance is attributableto a cause as described below. The powder of metal silicon or tin or analloy thereof electrochemically forms an alloy with lithium during thecharging operation but the alloy formation does not take placeuniformly. Thus, the expansion due to the lithium alloy formation occursunevenly so that “voids” and cracks generates near the surface of and inthe active material layer and in a region of the interface of the activematerial layer and the current collector. As “voids” and cracks appear,the electron conduction in the active material layer of the negativeelectrode is blocked to increase the electric resistance of theelectrode. This is probably attributable to the very large expansion atthe time of the formation of the alloy of lithium and powder of metalsilicon or tin or an alloy thereof. Although the expansion at the timeof the formation of the lithium alloy can be suppressed by reducing alithium amount for forming an alloy, the capacity that the battery canstore is also reduced.

Thus, in order to prevent the generation of “voids” and cracks, theinventors of the present invention devised an electrode structure by (1)dispersing a support material that can maintain electron conduction evenwhen the powder of metal silicon or tin or an alloy thereof in theactive material layer expands, (2) compounding the binder with aconductive organic polymer to improve the electron conduction of thebinder, (3) arranging a conductive buffer layer between the currentcollector and the active material layer that can ensure electronconduction between the active material layer and the current collectoreven when the powder of metal silicon or tin or an alloy thereof in theactive material layer expands, and (4) arranging a surface coat layershowing expansion less than the metal silicon or tin or an alloy thereofin the active material layer and also ion transmissibility and electronconductivity in order to suppress the unevenness of expansion at thesurface of the electrode layer.

Now, embodiments of the present invention will be described below withreferring to FIGS. 1A through 5B.

Referring to FIGS. 1A and 1B, there are shown a current collector 100,particles of silicon or tin or an alloy containing either of theelements 101, particles of an auxiliary conductive material 102, abinder 103, particles of a support material 104, a main active materiallayer 105 and an electrode structure 106. In FIG. 1B, referencecharacter 101′ denotes particles of silicon or tin or an alloycontaining either of the elements that expanded as a result of formingan alloy with lithium.

Now, referring to FIG. 2, there are shown a current collector 200,particles of silicon or tin or an alloy containing either of theelements 201, particles of an auxiliary conductive material 202, abinder 203, particles of a support material 204, a main active materiallayer 205, electron conducting particles 206, a binder 207, a bufferlayer 208, and an electrode structure 209.

Referring to FIG. 3, there are shown a current collector 300, particlesof silicon or tin or an alloy containing either of the elements 301,particles of an auxiliary conductive material 302, a binder 303,particles of a support material 304, a main active material layer 305,ion conducting or electron conducting particles 306, a binder 307, alithium-ion-transmitting surface coat layer 308, and an electrodestructure 309.

Referring then to FIGS. 4A and 4B, there are shown a current collector400, particles of silicon or tin or an alloy containing either of theelements 401, particles of an auxiliary conductive material 402, abinder 403, particles of a support material 404, a main active materiallayer 405, electron conducting particles 406, a binder 407, a bufferlayer 408, ion conducting or electron conducting particles 409, a binder410, a lithium-ion-transmitting surface coat layer 411, an electrodestructure 412, and a surface-coarsened current collector 413.

Referring finally to FIGS. 5A and 5B, there are shown a currentcollector 500, particles of silicon or tin or an alloy containing eitherof the elements 501, flat particles of an auxiliary conductive material502, particles of an auxiliary conductive material 503, a binder 504, amain active material layer 505, an electrode structure 506, andparticles of silicon or tin or an alloy containing either of theelements that expanded as a result of forming an alloy with lithium501′.

Thus, FIG. 1A is a schematic cross-sectional view of an embodiment ofelectrode structure 106 formed by introducing a support material 104,which has particle sizes comparable to the thickness of the main activematerial layer 105 and assists the electron conduction in vertical andhorizontal directions in the main active material layer, into the mainactive material layer 105 that is formed by particles of silicon or tinor an alloy of either of them 101, particles of an auxiliary conductivematerial 102 (for assisting electron conduction among particles) and abinder 103. As the technique for introducing the support material 104,the powder of the support material is added to and mixed with the powderof metal silicon or tin or an alloy of either of them, the powder of theauxiliary conductive material and the binder at the time of forming amain active material layer on the current collector.

FIG. 1B is an imaginary schematic cross-sectional view of the electrodestructure of FIG. 1A when the lithium secondary battery prepared bycombining the electrode structure that operates as a negative electrodewith a positive electrode is electrically charged and lithium forms analloy with particles of silicon or tin or an alloy of either of them asa result of electrochemical reaction to expand the particles.

FIG. 5A is a schematic cross-sectional view of an embodiment ofelectrode structure in which the active material layer is formed byintroducing flat particles of an auxiliary conductive material in placeof the support material of FIG. 1A. FIG. 5B is an imaginary schematiccross-sectional view of the electrode structure of FIG. 5A when thelithium secondary battery prepared by combining the electrode structurethat operates as a negative electrode with a positive electrode iselectrically charged and lithium forms an alloy with particles ofsilicon or tin or an alloy of either of them as a result ofelectrochemical reaction to expand the particles.

By comparing FIG. 1B and FIG. 5B, it will be seen that the fall ofelectron conduction is reduced and hence the rise of electric resistanceis suppressed at the time of electrode reaction due to the presence ofparticles of a support material 104 in the electrode structure of FIG.1A in comparison with the electrode structure of FIG. 5A. Additionally,the charging-discharging efficiency (Coulombic efficiency) is improvedand the cycle life is prolonged when a conductive polymer is compoundedwith the binder of FIG. 18 because electron conduction is maintained bymeans of the network of the binder (not shown).

FIG. 2 is a schematic cross-sectional view of an embodiment of electrodestructure according to the present invention, where an electronconducting buffer layer is arranged between the current collector andthe main active material layer of the electrode structure. When theelectrode structure of FIG. 2 is used as the negative electrode of alithium secondary battery, the electron conduction between the mainactive material layer, which expands at the time of charging operation,and the current collector is assisted by the buffer layer that expandsonly to a small extent. Additionally, unlike the electrode structure ofFIG. 1A, the stress of the interface between the current collector andthe main active material layer is alleviated and distributed uniformlybecause the buffer layer that is made of a binder of an organic polymeris arranged between the current collector and the main active materiallayer. Thus, the main active material layer is prevented from peelingoff from the current collector, if partly, and the deformation of thecurrent collector that is produced by stress is suppressed. Preferably,the binder of the main active material layer and that of the bufferlayer are made of the same materials or material(s) of the same qualitybecause then the interface of the buffer layer and the main activematerial layer is formed continuously.

FIG. 3 is a schematic cross-sectional view of another embodiment ofelectrode structure according to the present invention formed byarranging a surface coat layer on the surface of the main activematerial layer of an electrode structure as shown in FIG. 1A. When alithium secondary battery is formed by using an electrode structure asshown in FIG. 3 as a negative electrode and charged, the expansion ofthe main active material layer is made uniform and the electronconduction parallel to the current collector is apt to be maintained atthe time of electrochemical reaction because the surface coat layer 306that expands only to a small extent by charging and shows electronconductivity is arranged to uniformize the electric field intensity thatis applied to the surface of the electrode structure of FIG. 3 at thetime of charging. Thus, the electric resistance of the electrodestructure can be minimized when it expands by charging.

FIGS. 4A and 4B are schematic cross-sectional views of still anotherembodiment of electrode structure according to the present invention, inwhich an electron conducting buffer layer is arranged between thecurrent collector and the main active material layer and a surface coatlayer is arranged on the surface of the main active material layer. FIG.4A shows that the embodiment includes a current collector whose surfaceis planar, while FIG. 4B shows that the embodiment includes a currentcollector whose surface is coarsened. When a current collector having acoarsened surface is used, the area of the interface thereof isincreased to reduce the stress that arises between the main activematerial layer and the current collector at the time ofcharging/discharging. However, fissures can appear in the currentcollector as the charging/discharging cycle of the lithium secondarybattery is repeated if the undulations of the surface are large and notuniform because stress is not applied uniformly to the currentcollector. Therefore, the ten point average height Rz showing thesurface coarseness of the current collector is preferably 0.5 μm to 5μm, more preferably 0.7 μm to 3 μm.

[Main Active Material Layer]

For the main active material layer of an electrode structure to be usedas the negative electrode of a lithium secondary battery, firstly a mainactive material, a powder of a support material, a powder of anauxiliary conductive material and a binder are mixed, and optionally asolvent for the binder is added to the mixture, which mixture is thenkneaded to prepare slurry of the mixture. Subsequently, the preparedslurry is applied onto a current collector or a buffer layer, which willbe described later, and dried to form an electrode layer, which isoptionally subjected to a press process to regulate the thickness andthe density of the electrode layer to produce an electrode structure.Typical techniques that can be used to apply the slurry for the purposeof the present invention include a coater application method and ascreen printing method. Alternatively, it is also possible to form anelectrode material layer by molding a main material, the powder of thesupport material, the powder of the auxiliary conductive material andthe binder on a current collector or a buffer layer without adding asolvent under pressure. For the purpose of the present invention, thedensity of the electrode material layer is preferably within a rangefrom 0.7 to 2.0 g/cm³ more preferably within a range from 0.9 to 1.5g/cm³. If the density of the electrode material layer is too large, theexpansion of the electrode layer becomes excessive at the time oflithium introduction to allow the electrode layer to peel off from thecurrent collector. If, on the other hand, the density of the electrodematerial layer is too small, the resistance of the electrode becomeslarge to by turn reduce the charging-discharging efficiency and producea large voltage fall at the time of discharging the battery.

(Main Active Material)

Metal particles of silicon, tin or an alloy thereof are preferably usedfor the main active material of the main active material layer.

Preferably, the metal particles (alloy particles) contain a transitionmetal element and are compounded with carbon. Preferably, the metalparticles (alloy particles) have a crystal size of 60 nm or less. Morepreferably, they are amorphous particles having a size of 20 nm or less.For the purpose of the present invention, the crystal size of particlesis determined from the half width of the peak of the X-ray diffractioncurve and the diffraction angle obtained by using an X-ray radiationsource of CuKα and also the Scherrer's formula shown below:Lc=0.94λ/(β cos θ)  (Scherrer's formula),Wherein Lc: crystal size

-   -   λ: wavelength of X-ray beam    -   β: half width of the peak (radian)    -   θ: Bragg angle of diffraction line.

The average particle size of the primary particles of the metalparticles (alloy particles) of the main active material is preferablywithin a range from 0.02 μm to 5 μm, more preferably within a range from0.1 μm to 3 μm.

(Coating of Metal Powder Selected from Silicon, Tin or an Alloy Thereofand Capable of Storing and Discharging Lithium by ElectrochemicalReaction)

Preferably, the particles of silicon, tin or an alloy thereof are coatedwith a material selected from pitch, pitch coke, petroleum coke, coaltar, fluoranthene, pyrene, chrysene, phenanthrene, anthracene,naphthalin, fluorene, biphenyl and acenaphthene and subsequentlycarbonized in an inert gas atmosphere so that the particles are coatedwith a carbonized layer. Of the above listed materials to be carbonized,pitch, pitch coke, petroleum coke and coal tar provide an advantage ofeasiness of coating particles of the main active material because theyshow a low melting point. Additionally, they also provide an advantageof a low carbonization temperature that facilitates the operation ofcoating with a carbonized layer without reducing the lithium occlusionperformance of particles of the main active material. Stilladditionally, they are inexpensive. The ratio of the carbonized coatlayer in the particles of silicon, tin or an alloy thereof is preferablywithin a range from 1 to 10 wt %, more preferably within a range from 2to 5 wt % from the viewpoint of not sacrificing the lithium occlusioncapacity of silicon, tin or an alloy thereof, assisting theinter-particle electron conduction and suppressing oxidation. Areduction reaction of depriving oxygen atoms from the partly oxidizedparticles of silicon, tin or an alloy thereof takes place in theabove-described carbonization process to reduce the quantity of theoxide that gives rise to an irreversible reaction with lithium. Theinert gas that is used in the carbonization process is selected fromargon gas, nitrogen gas and carbon dioxide gas. The carbonizationtemperature is preferably within a range from 400° C. to 900° C., morepreferably within a range from 500° C. to 800° C. because carbonizationis accelerated while crystallization of silicon, tin or an alloy thereofis retarded within the temperature range.

The metal powder selected from silicon, tin or an alloy thereof andcapable of storing and discharging lithium by electrochemical reactionare preferably a powder of particles having a coating on a part of thesurface or the whole surface, the coating being composed of TiO₂, MoO₃or WO₃ or an oxide which is formed by substituting a part of Ti, Mo or Wof these metal oxides by another metal element. Because theabove-described metal oxides have a stable structure and easily carryout the electrical insertion and elimination of lithium ion, in thelithium secondary battery including the negative electrode mainlycomposed of a metal powder selected from silicon, tin or an alloythereof, it is possible to prevent lithium ions from depositing asreduced and active lithium metal on the metal powder selected fromsilicon, tin or an alloy thereof. As the result, reduction of thecharging/discharging performance due to repetition of charging anddischarging of the battery can be prevented to prolong the cycle life.

The coating on the metal oxide can be formed by mixing the metal powderselected from silicon, tin or an alloy thereof with the above-describedmetal oxide in a grinder such as a ball mill. Also, it is possible toform the coating by mixing the metal powder selected from silicon, tinor an alloy thereof with a solution of polytitanic acid, polytungsticacid, polymolybdic acid, polytitanic acid peroxide, polytungstic acidperoxide, polymolybdic acid peroxide which are the raw material of themetal oxide.

Further, it is preferable to coat a part of surface or the whole surfaceof the metal powder selected from silicon, tin or an alloy thereof witha conductive polymer having conjugated double bonds of carbon-carbonwhere single bond and double bond alternate. When the negative electrodeof a lithium secondary battery is formed using a binder, this coatingmakes it possible to increase the affinity of the binder and the metalpowder selected from silicon, tin or an alloy thereof and uniformlydisperse them in the electrode layer of the negative electrode to obtaina stable negative electrode performance.

Furthermore, lithium ions can be electrically inserted into andeliminated from the conductive polymer and therefore the same effect asof the metal oxide can be obtained. As the conductive polymer, a polymerobtained by polymerizing thiophene derivatives, pyrrole derivatives,aniline derivatives, acetylene derivatives, or the like. Additionally,the conductive polymer is more preferably a composite compound with asurfactant in order to obtain affinity of the conductive polymer and thebinder. The coating of the conductive polymer can be formed by mixingthe metal powder with a solution of the conductive polymer.

(Support Material)

The support material of the main active material layer is preferably oneor more materials selected from the group consisting of graphite, anoxide of a transitional metal (which is selected from TiO₂, MoO₃ andWO₃), a metal electrochemically not forming an alloy with Li (which isselected from the group consisting of Cu, Ni, Co, Ti, Fe, Cr, Mo, W, Pd,Ft and Au and alloys thereof). More preferably, the support material isselected from graphite and oxides of a transitional metal (selected fromTiO₂, MoO₃ and WO₃) from the viewpoint of suppressing the growth ofdendrite of lithium in the charging operation. Graphite and oxides oftransitional metals selected from TiO₂, MoO₃ and WO₃ can occlude lithiumbetween layers thereof. Graphite and oxides of a transitional metalselected from MoO₃ and WO₃ are further more preferable because of asmall potential difference between them and metal lithium at the time ofoccluding lithium. Among the above listed materials, graphite is mostpreferable as the support material to be used in the present inventionbecause it can hold electrolyte solution. For the purpose of the presentinvention, the powdery supporting material is provided preferably asparticles having a spherical, pseudo-spherical or pillar shape. Theaverage size of particle (secondary particles) of the support materialis preferably from 0.3 to 1.35 times, more preferably from 0.6 to 1.2times of the average thickness of the main active material layer for thepurpose of ensuring electron conduction in the main active materiallayer when the min active material layer expands in the chargingoperation.

(Auxiliary Conductive Material)

Materials that can be used for the auxiliary conductive material of themain active material layer include amorphous carbon materials such asacetylene black, and ketjen black; carbon materials such as a graphitestructure carbon; nickel, copper, silver, titanium, platinum, aluminum,cobalt, iron and chromium, and particularly graphite is preferablebecause it can hold electrolyte solution and shows electron conductivityand a large specific surface area. The preferable shape of particles ofthe auxiliary conductive material may be spherical, flake-shaped,filament-shaped, fiber-shaped, spike-shaped or needle-shaped. It ispossible to raise the packing density and lower the impedance of theelectrode structure by using powder of the auxiliary conductive materialhaving two or more different shapes when forming the electrode materiallayer. The average size of particle (secondary particles) of theauxiliary conductive material is preferably 10 μm or less, morepreferably 5 μm or less.

As the auxiliary conductive material for use in the negative electrodeof a lithium secondary battery, which is mainly composed of a metalpowder selected from silicon, tin or an alloy thereof and capable ofstoring and discharging lithium by electrochemical reaction, a metalpowder having characteristics of so-called super elastic alloys whichare obtained by heat-treating the powder of a intermetallic compound ofNi—Ti (for example, at a temperature of 480° C.) is preferably used. Inthe lithium secondary battery including a negative electrode composed ofa metal powder selected from silicon, tin or an alloy thereof, anelectric amount to be charged is large and a negative electrode expandsand contracts during charging and discharging, but use of theabove-described auxiliary conductive material in the negative electrodemakes it possible to prevent reduction of the collecting performance ofthe negative electrode due to the expansion and contraction of thenegative electrode, thereby preventing reduction of battery performancedue to repetition of charging and discharging.

(Binder)

Materials that can be used for the binder of the main active materiallayer include organic polymer materials such as polyimideamides,polyimides, polyimide precursors (polyamic acid before polyimideformation, or imperfectly formed polyimides), styrene-butadiene rubber,and denatured polyvinyl alcohol-based resins with reducedwater-absorbing property. Particularly, a polyimide precursor (polyamicacid before polyimide formation, or an imperfectly formed polyimide) ispreferably used for binding silicon alloy powder and turned to a perfectpolyimide by heat-treating it at 150 to 300° C. after application of theelectrode layer.

The content of the binder in the main active material layer ispreferably 2 to 20 wt %, more preferably 5 to 10 wt %.

Since the organic polymers to be used for the binder such aspolyimideamides, polyimides, polyimide precursors (polyamic acid beforepolyimide formation, or imperfectly formed polyimides),styrene-butadiene rubber, and denatured polyvinyl alcohol-based resinswith reduced water-absorbing property are poorly electron conductive,any of them is preferably used by adding a conductive polymer havingalternately arranged carbon-carbon double bonds and single bonds of—C═C—C═C— to reduce the resistance of the electrode. Since conductivepolymers show a strong affinity for any of the above listed organicpolymers, such a conductive polymer added to the organic polymer makesit possible to realize more uniform composite material formation. Theconductive polymer is added to the organic polymer of the binderpreferably at an amount from 1 to 20 based on 100 of the binder byweight from the viewpoint of maintaining the binding force and reducingthe electric resistance. The conductive polymer is added to the organicpolymer more preferably at an amount from 2 to 10 based on 100 of thebinder by weight in order to raise the electric conductivity withoutreducing the binding force of the binder. Conductive polymers that canbe used for the purpose of the present invention include polymers ofthiophene derivative monomers, pyrrole derivative monomers, anilinederivative monomers, acetylene derivative monomers and phenylenederivative monomers. Specific examples of conductive polymers includepolythiophene, poly(3-hexylthiophene), poly(2-acetylthiophene),polybenzothiopnene, poly(2,5-dimethylthiophene), poly(2-ethylthiophene),poly(3-carboxylic ethyl thiophene), polythiopheneacetonitrile,poly(3,4-ethylenedioxythiophene), polyisothianaphthene, polypyrrole,polyaniline and polyparaphenylene. When adding the conductive polymer tothe organic polymer of the binder, it is added to the binder beforemixing it with the active material and the conductive polymer in orderto raise the electron conductivity of the binder more effectively.

[Current Collector]

The current collector of an electrode structure according to the presentinvention takes the role of efficiently supplying an electric current tobe consumed in the electrode reaction in the charging operation orcollecting the electric current that is generated at the time ofdischarging operation.

Particularly, when an electrode structure according to the presentinvention is applied to the negative electrode of a secondary battery,the use of a material that shows a high electric conductivity and isinactive relative to battery reactions is preferable. Preferablematerials that can be used for the current collector include metalmaterials formed of one or more metals selected from copper, nickel,iron, stainless steel, titanium and platinum. The use of copper that isinexpensive and shows a low electric resistance is highly preferable.While the current collector has a “plate shape”, the thickness of theplate shape of the current collector is not particularly limited so longas it is good for practical applications. In other words, the currentcollector may include the form of “foil” having a thickness of about 5μm to 100 μm. Additionally, the plate may include a mesh-like plate, asponge-like plate, a fiber-like plate, a punched metal plate and anexpanded metal plate so long as it has a “plate shape”.

As for the surface roughness of the current collector, the ten pointaverage height Rz of the current collector is preferably within a rangefrom 0.5 μm to 5.0 μm, more preferably within a range from 0.7 μm to 3.0μm in order to maintain the tight adhesion of the electrode layer formedon the current collector. If Rz is greater than the upper limit of theabove range, the electrode layer formed on the current collector showsan uneven thickness and can give rise to fissures due to the stressattributable to the expansion/contraction of the main active materiallayer when it is installed in a lithium secondary battery and subjectedto charging/discharging operations. If, on the other hand, Rz is smallerthan the lower limit of the above range, the main active material layercan become easily separated from the current collector along theinterface due to the stress attributable to the expansion/contraction ofthe main active material layer when it is installed in a lithiumsecondary battery and subjected to charging/discharging operations,

[Buffer Layer]

According to the present invention, the buffer layer is arranged betweenthe current collector and the main active material layer and includes atleast a binder of an organic polymer, and particles of one or moreconductive materials selected from the group consisting of a conductivepolymer, graphite, a metal electrochemically not forming any alloy withLi (which is selected from the group consisting of Cu, Ni, Co, Ti, Fe,Cr, Mo, W, Pd, Pt and Au and alloys thereof) and an oxide of atransitional metal (which is selected from TiO₂, MoO₃ and WO₃).

For the buffer layer, firstly a powdery conductive material and a binderare mixed, and optionally a solvent for the binder is added to themixture, which is then kneaded to prepare slurry of the mixture.Subsequently, the prepared slurry is applied onto a current collectorand dried to form an electrode layer, which is optionally subjected to apress process, and regulate the thickness and the density of theelectrode layer to produce an electrode structure. Typical techniquesthat can be used to apply the slurry for the purpose of the presentinvention include a coater application method and a screen printingmethod. Alternatively, it is also possible to form an electrode materiallayer by molding a powdery conductive material and a binder on a currentcollector without adding a solvent under pressure. For the purpose ofthe present invention, the density of the electrode material layer ispreferably within a range from 0.7 to 2.0 g/cm³, more preferably withina range from 0.9 to 1.5 g/cm³.

The average particle size of the particles of the conductive material ispreferably 0.5 μm to 10 μm from the viewpoint of forming the bufferlayer with a uniform thickness. If the thickness of the buffer layer ismade large, (1) the thickness of the overall electrode layer is madelarge to reduce the storage capacity of the electrode layer and (2) theexpansion coefficient of the buffer layer and that of the main activematerial layer shows a large difference at the time of chargingoperation to produce a large stress, which warps the buffer layer andforces it to peel off.

The binder of the buffer layer and the binder of the active materiallayer are preferably made of the same material(s) or material(s) of thesame quality because then an interface is hardly formed between thebuffer layer and the main active material layer. If an interface isformed and a stress is produced, the buffer layer can easily peel offfrom the interface. Materials that can be used for the binder of thebuffer layer include organic polymer materials such as polyimideamides,polyimides, polyimide precursors (before polyimide formation, orimperfectly formed polyimides), styrene-butadiene rubber, and denaturedpolyvinyl alcohol-based resins with reduced water-absorbing propertysimilarly as in the case of the main active material layer. The contentof the binder in the buffer layer is preferably 2 to 20 wt %, morepreferably 5 to 10 wt %. The above listed organic polymer materials areelectrically highly insulating and hence it is preferable to mix aconductive polymer material with it in order to raise theelectro-conductivity of the buffer layer. Then, it is possible toachieve a highly uniform composite-material formation because conductivepolymer has a strong affinity for an organic polymer. The conductivepolymer is added to the organic polymer of the binder preferably at anamount from 1 to 20, more preferably at amount from 2 to 10, based on100 of the binder by weight from the viewpoint of maintaining thebinding force and reducing the electric resistance. Conductive polymersthat can be used for the purpose of the present invention includepolymers of thiophene derivative monomers, pyrrole derivative monomers,aniline derivative monomers, acetylene derivative monomers and phenylenederivative monomers.

[Surface Coat Layer]

According to the present invention, the surface coat layer is arrangedon the surface of the main active material layer and has electronconductivity and an ion transmissibility (ionic conductivity). Thesurface coat layer includes at least a binder of an organic polymer, andparticles of one or more materials selected from the group consisting ofa conductive polymer, graphite, a metal electrochemically not forming analloy with Li (which is selected from the group consisting of Cu, Ni,Co, Ti, Fe, Cr, Mo, W, Pd, Pt and Au and alloys thereof), and oxides ofa transitional metal (which is selected from TiO₂, MoO₃ and WO₃).Graphite and oxides of a transitional metal selected from TiO₂, MoO₃ andWO₃ are preferable as materials for the particles because they canintercalate lithium for storage. Graphite and oxides of a transitionalmetal selected from MoO₃ and WO3 are more preferable because they show asmall potential difference compared with metal lithium at the time ofintercalating lithium. The average particle size of the secondaryparticles of the particles is preferably 0.5 μm to 10 μm from theviewpoint of forming the electrode layer of the electrode structure witha uniform thickness. If the thickness of the coat layer is made large,the thickness of the overall electrode layer is made large to reduce thestorage capacity of the electrode layer.

The binder of the buffer layer and the binder of the active materiallayer are preferably made of the same material(s) or material(s) of thesame quality because then an interface is hardly formed between thebuffer layer and the main active material layer. If an interface isformed and a stress is produced, the buffer layer can easily peel offfrom the interface. Materials that can be used for the binder of thebuffer layer include organic polymer materials such as polyimideamides,polyimides, polyimide precursors (before polyimide formation, orimperfectly formed polyimides), styrene-butadiene rubber, and denaturedpolyvinyl alcohol-based resins with reduced water-absorbing propertysimilarly as in the case of the main active material layer. The contentof the binder in the buffer layer is preferably 2 to 20 wt %, morepreferably 5 to 10 wt %. The above listed organic polymer materials areelectrically highly insulating and therefore any of them is preferablyused by adding a conductive polymer having alternately arrangedcarbon-carbon double bonds and single bonds of —C═C—C═C— to raise theelectron conductivity. Since conductive polymers show a strong affinityfor any of the above listed organic polymers, such a conductive polymeradded to the organic polymer makes it possible to realize more uniformcomposite material formation. The conductive polymer is added to theorganic polymer of the binder at an amount of preferably 1 to 20, morepreferably 2 to 10, based on 100 of the binder by weight from theviewpoint of maintaining the binding force and reducing the electricresistance, Conductive polymers that can be used for the purpose of thepresent invention include polymers of thiophene derivative monomers,pyrrole derivative monomers, aniline derivative monomers, acetylenederivative monomers and phenylene derivative monomers.

[Secondary Battery]

A secondary battery according to the invention includes a negativeelectrode formed by using an electrode structure having theabove-described features, an electrolyte and a positive electrode, andutilizes the oxidation reaction of lithium and the reduction reaction oflithium ions. FIG. 6 is a schematic cross-sectional view of anembodiment of lithium secondary battery according to the presentinvention, illustrating the basic configuration thereof. Referring toFIG. 6, the secondary battery includes a negative electrode 601 formedby using an electrode structure according to the present invention, anion conductor 602, a positive electrode 603, a negative electrodeterminal 604, a positive electrode terminal 605, and a battery jar(housing) 606.

To prepare the secondary battery, an electrode group is formed as astacked-layer structure by sandwiching the ion conductor between thenegative electrode and the positive electrode, and then te electrodegroup is inserted into the battery jar under dry air or under a dryinert gas atmosphere where the dew point temperature is carefullycontrolled. Subsequently, the electrodes and the electrode terminals areconnected and the battery jar is hermetically sealed to complete theoperation of assembling the battery. When a porous polymer film holdingthe electrolyte solution is used as the ion conductor, the electrodegroup is formed by sandwiching the porous polymer film between thenegative electrode and the positive electrode as a separator forpreventing short-circuiting and then inserted into the battery jar.Subsequently, the electrodes and the electrode terminals are connected,and the electrolyte solution is injected before the battery jar ishermetically sealed to complete the operation of assembling the battery.

A lithium secondary battery including a negative electrode formed byusing an electrode structure of an electrode material according to thepresent invention shows a high capacity and a high energy density andhas a satisfactory cycle life because of the above-described remarkableadvantages of the negative electrode.

(Positive Electrode 602)

The positive electrode 602 of the lithium secondary battery includingthe negative electrode of the electrode structure according to thepresent invention that is arranged opposite to the negative electrode ismade of an positive electrode material that operates at least as lithiumion source and is a host material of lithium ions. Preferably, thepositive electrode 602 is composed of a layer formed by using anpositive electrode material that is a host material of lithium ions anda current collector. Additionally, the layer formed by using a positiveelectrode material includes the positive electrode material that is ahost material of lithium ions and a binder, to which an auxiliaryconductive material is added in certain cases.

Preferable positive electrode materials that function as lithium ionsources and are host materials for a lithium secondary battery accordingto the invention include lithium-transition metal oxides,lithium-transition metal sulfides, lithium-transition metal nitrides andlithium-transition metal phosphorylated compounds. Transition metals ofsuch transition metal oxides, transition metal sulfides, transitionmetal nitride and transition metal phosphorylated compounds includemetal elements typically having a d shell or an f shell. Examples ofsuch transition metal elements include Sc, Y, lanthanoid, actinoid, Ti,Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni,Pb, Pt, Cu, Ag and Au, and particularly Co, Ni, Mn, Fe, Cr and Ti arepreferable.

Use of lithium-transition metal oxides, lithium-transition metalsulfides, lithium-transition metal nitrides and lithium-transition metalphosphorylated compounds makes it possible to exploit the performance ofthe negative electrode of an electrode structure according to thepresent invention and design the battery voltage and the storagecapacity so as to obtain a high energy density lithium secondarybattery, by mixing a plurality of positive electrode materials obtainedby appropriately selecting transition metal elements contained in theabove listed compounds.

When the positive electrode active materials are in a powdery form, alayer of the positive electrode active materials is formed on thecurrent collector by using a binder or sintering or evaporating thematerials to prepare the positive electrode. If the powder of thepositive electrode active materials is poorly conductive, it will benecessary to mix an auxiliary conductive material as in the case offorming the active material layer of the electrode structure.Preferably, the powder of the positive electrode active materials arecoated with a material selected from pitch, pitch coke, petroleum coke,coal tar, fluoranthene, pyrene, chrysene, phenanthrene, anthracene,naphthalin, fluororene, biphenyl and acenaphthene and subsequentlycarbonized under an inert gas atmosphere so that the powder is coatedwith a carbonized layer. Of the above listed materials to be carbonized,pitch, pitch coke, petroleum coke and coal tar provide an advantage ofeasiness of coating particles of the positive electrode materialsbecause they show a low melting point. Additionally, they also providean advantage of a low carbonization temperature that facilitates theoperation of coating with a carbonized layer without reducing theperformance of the positive electrode materials. The contact resistancebetween the particles of the positive electrode materials can bereduced. Still additionally, they are inexpensive. The ratio of thecarbonized coat layer is preferably within a range from 1 to 5 wt % fromthe viewpoint of not sacrificing the lithium occlusion capacity of thepositive electrode active materials and assisting the inter-particleelectron conduction. Materials that can be used for the auxiliaryconductive material and the binder include fluorine resins such aspolyvinylidene fluoride, polyolefin resins such as polyethylene, rubbertype resins such as styrene-butadiene rubber, polyimideamides,polyimides, polyimide precursors (before polyimides formation, orimperfectly formed polyimides), and denatured polyvinyl alcohol-basedresins with reduced water-absorbing property.

Since above-described binder is electrically highly insulating and henceit is preferable to mix a conductive polymer material with the binder inorder to raise the electro-conductivity of the positive electrode. Then,it is possible to achieve a highly uniform composite material formationbecause a conductive polymer has a strong affinity for organic polymer.The conductive polymer is added to the organic polymer of the binderpreferably at an amount of 1 to 20, more preferably 2 to 10, based on100 of the binder by weight from the viewpoint of maintaining thebinding force and reducing the electric resistance. Conductive polymersthat can be used for the purpose of the present invention includepolymers of thiophene derivative monomers, pyrrole derivative monomers,aniline derivative monomers, acetylene derivative monomers and phenylenederivative monomers.

Materials that can be used for the current collector of the positiveelectrode include aluminum, titanium, nickel and platinum that show ahigh electric conductivity and are inactive relative to batteryreactions. More specifically, nickel, stainless, titanium and aluminumare preferable. Above all, aluminum is highly preferable because it isinexpensive and shows a high electric conductivity. Although the currentcollector has a “plate shape”, the thickness of the plate shape of thecurrent collector is not particularly limited so long as it is good forpractical applications. In other words, the current collector may beformed in the form of “foil” having a thickness of about 5 μm to 100 μm.Additionally, the plate may include a mesh-like plate, a sponge-likeplate, a fiber-like plate, a punched metal plate and an expanded metalplate so long as it has a “plate shape”.

(Ion Conductor 603)

Conductors of lithium ions that can be used for the ion conductor of alithium secondary battery according to present invention include aseparator holding an electrolyte solutions (prepared by dissolving anelectrolyte in a solvent), a solid electrolyte, a solidified electrolyteobtained by forming gel of an electrolyte solution by a polymeric gel orthe like, and a composite of a polymeric gel and a solid electrolyte.

The electric conductivity of the ion conductor to be used for asecondary battery according to the present invention is preferably1×10⁻³ S/cm or more, more preferably 5×10⁻³ S/cm or more, at 25° C.

Electrolytes that can be used for the ion conductor include salts oflithium ion (Li⁺) and Lewis acid ions (BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, ClO₄ ⁻,CF₃SO₃ ⁻ and BPh₄ ⁻ (Ph: phenyl group)) and mixed salts thereof. It isdesirable that the salt to be used for the ion conductor is sufficientlydehydrated and deoxygenated by heating under reduced pressure.Additionally, electrolytes prepared by dissolving lithium salt in fusedsalt may also be used for the purpose of the present invention.

Solvents that can be used for dissolving the electrolyte of the ionconductor include acetonitrile, benzonitrile, propylene carbonate,ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethylcarbonate, dimethylformamide, tetrahydrofuran, nitrobenzene,dichloroethane, diethoxyethane, 1,2-dimethoxyethane, chlorobenzene,γ-butyrolactone, dioxolane, sulfolane, nitromethane, dimethyl sulfide,dimethyl suloxide, methyl formate, 3-methyl-2-oxazolidinone,2-methyltetrahydrofurane, 3-propylsydonone, sulfur dioxide, phosphorylchloride, thionyl chloride, sulfuryl chloride and mixed solutionsthereof.

The above solvents are preferably dehydrated by active alumina, amolecular sieve, phosphorus pentoxide or calcium chloride. Depending onthe type of solvent to be used, the solvent may be distilled in thepresence of alkali metal in inert gas to remove impurities and dehydratethe solvent. The electrolyte concentration of the electrolyte solutionprepared by dissolving the electrolyte into the solvent is preferablywithin a range from 0.5 to 2.0 mol/liter for the purpose of realizing ahigh ionic conductivity.

Preferably, vinyl monomer that is apt to give rise to an electrolyticpolymeric reaction may be added to the electrolyte solution for thepurpose of suppressing the reaction between the electrode and theelectrolyte solution. As vinyl monomers is added to the electrolytesolution, polymer coat film is formed on the surfaces of silicon alloyparticles of the main active material by the charging reaction of thebattery to suppress the reaction between the lithium that is occluded bysilicon alloy particles or deposited on the surfaces of silicon alloyparticles and the organic solvent of the electrolyte solution at thetime of charging operation and consequently prolong the cycle life ofthe battery. When the vinyl monomer is only slightly added to theelectrolyte solution, the above-described effect does not arise. Whenthe vinyl monomer is added to the electrolyte solution is too much, theionic conductivity of the electrolyte solution is reduced and thepolymer coat film formed at the time of charging operation becomes largeto increase the resistance of the electrode. Therefore, the vinylmonomer added to the electrolyte solution is preferably within a rangefrom 1 to 5 wt %.

Specific examples of vinyl monomers that can be used for the purpose ofthe present invention include styrene, 2-vinylnaphthalene,2-vinylpyridine, N-vinyl-2-pyrrolidone, divinyl ether, ethyl vinylether, vinylphenylether, methyl methacrylate, methyl acrylate,acrylonitrile and vinyl carbonate, and particularly, 2-vinylnaphthalene,2-vinylpyridine, N-vinyl-2-pyrrolidone, divinyl ether, ethyl vinylether, vinylphenylether and vinyl carbonate are more preferable. Vinylmonomers having one or more aromatic groups are preferable because suchmonomers show a strong affinity for lithium ions. The use of combinationof N-vinyl-2-pyrrolidone, divinyl ether, ethyl vinyl ether,vinylphenylether or vinyl carbonate that shows a strong affinity tosolvents of the electrolyte solution with a vinyl monomer having one ormore aromatic groups is more preferable for the purpose of the presentinvention.

From the viewpoint of preventing electrolyte solution from leaking, theuse of solid electrolyte or solidified electrolyte is preferable.Examples of solid electrolyte include glass containing oxides formedfrom lithium, silicon, oxygen and phosphor or sulfur, and polymercomplexes of organic polymers having an ether structure. Preferableexamples of solidified electrolyte include those obtained by forming gelof the electrolyte solution by a gelling agent. Preferable gel formingagents that can be used for the purpose of the present invention includepolymers that swell by absorbing the solvent of the electrolyte solutionand porous materials that absorb a large amount of liquid such as silicagel. Examples of such polymers include polyethylene oxide, polyvinylalcohol, polyacrylonitrile, polymethylmethacrylate, andvinylidenefluoride-hexafluoropropylene copolymer. Such polymerspreferably have a cross-linked structure.

When a separator is used, it takes a role of preventing short-circuitingfrom taking place between the negative electrode 601 and the positiveelectrode 602 in the secondary battery. It may also take the role ofholding the electrolyte solution. The separator is required to have alarge number of pores that allow lithium ions to move therethrough andto be insoluble to the electrolyte solution and stable. Examples ofmaterials that can be used for the separator include micro-porestructures and unwoven cloth such as glass, polyolefins includingpolypropylene and polyethylene, and fluorine resin. Additionally, metaloxide films having micro-pores and composites of resin films and suchmetal oxides can also be used for the purpose of the present invention.

[Profile and Structure of Battery]

As for the shape of a secondary battery according to the presentinvention, the battery has a flat, cylindrical, cubic or sheet shape. Asfor the structure of a secondary battery according to the presentinvention, the battery may be of a single layer type, a multilayer typeor a spiral type. Particularly, spiral type cylindrical batteriesprovide an advantageous feature of having a large electrode area becausea separator can be sandwiched between the negative electrode and thepositive electrode and wound and hence being able to flow a largeelectric current at the time of charging/discharging operations. Cubicor sheet-shaped batteries, on the other hand, provide an advantageousfeature of being adapted to be effectively stored in a narrow space ofappliance when the number of batteries to be stored is large.

Now, a secondary battery according to the present invention will bedescribed in detail in terms of shape and structure by referring toFIGS. 7 and 8. FIG. 7 is a schematic cross-sectional view of a singlelayered-flat type (coin type) battery and FIG. 8 is a schematiccross-sectional view of a spiral type cylindrical battery. The lithiumsecondary batterieshown in FIG. 7 or FIG. 8 are basically the same asthe battery of FIG. 6 in terms of configuration and include a negativeelectrode, a positive electrode, an ion conductor, a battery housing,and output terminals.

In FIGS. 7 and 8, there are shown negative electrodes 701 and 803,positive electrodes 703 and 806, negative electrode terminals (negativeelectrode cap and negative electrode tin) 704 and 808, positiveelectrode terminals (positive electrode tin and positive electrode cap)705 and 809, ion conductors 702 and 807, gaskets 706 and 810, a negativeelectrode current collector 801, a positive electrode current collector804, an insulating plate 811, a negative electrode lead 812, a positiveelectrode lead 813 and a safety valve 814.

In the case of the flat type (coin type) secondary batterieshown in FIG.7, the positive electrode 703 that includes a positive electrodematerial layer and the negative electrode 701 that has a negativeelectrode material layer are stacked through at least an ion conductor702 formed by using a separator holding electrolyte solution to form astaked body. The stacked body is contained in the positive electrode tin705, which operates as positive electrode terminal, from the positiveelectrode side, and the negative electrode side is covered by thenegative electrode cap 704 as negative electrode terminal. In theremaining part in the positive electrode tin, the gasket 706 isarranged.

In the case of the spiral type cylindrical secondary batterieshown inFIG. 8, the positive electrode 806 having an positive electrode(material) layer 805 formed on the positive electrode current collector804 and the negative electrode 804 having an electrode layer 802 formedon the negative electrode current collector 801 are arranged opposite toeach other through the ion conductor 807 formed by using a separatorholding electrolyte solution so as to form a stacked body having acylindrical structure formed by winding the positive electrode layer806, the negative electrode layer 802 and the ion conductor 807 by anumber of turns. Then, the stacked body of the cylindrical structure iscontained in the negative electrode tin 808 that operates as negativeelectrode terminal. The positive electrode cap 809 is arranged at theopen side of the negative electrode tin 808 to operate as positiveelectrode terminal. The gasket 810 is arranged in the remaining part ofthe negative electrode tin. The stacked body of the electrodes havingthe cylindrical structure is separated from the positive electrode capby the insulating plate 811. The positive electrode 806 is connected tothe positive electrode cap 809 by way of the positive electrode lead813. The negative electrode 803 is connected to the negative electrodetin 808 by way of the negative electrode lead 812. The safety valve 814is arranged at the side of the positive electrode cap to regulate theinternal pressure in the inside of the battery. The negative electrode803 is formed by using the above-described electrode structure accordingto the present invention.

Now, a method of assembling a battery as illustrated in FIG. 7 or FIG. 8will be described below.

-   (1) The separator (702, 807) is arranged between the negative    electrode (701, 803) and the molded positive electrode (703, 806)    and put into the positive electrode tin (705) or the negative    electrode tin (808).-   (2) After injecting the electrolyte solution, the negative electrode    cap (704) or the positive electrode cap (809) and the casket (706,    810) are assembled.-   (3) The assembly of the above (2) is caulked to complete the    operation of assembling the battery.

The operation of preparing the materials of the lithium battery and theoperation of assembling the battery are desirably conducted in dry airfrom which moisture is thoroughly eliminated or in dry inert gas.

Now, the members for forming a secondary battery according to thepresent invention and having the above-described configuration will bedescribed below.

(Gasket)

Examples of materials that can be used for the gasket include fluorineresin, polyolefin resin, polyamide resin, polysulfone resin and variousrubber materials. Techniques that can be used for sealing the batteryinclude the use of a sealed glass tube, the use of an adhesive agent,the use of welding and the use of soldering in addition to “caulking”involving the use of a gasket as shown in FIGS. 7 and 8. Any of variousorganic resin materials and ceramic materials may be used for theinsulating plate (811) of FIG. 8.

(Outer Tin)

The outer tin of the battery includes the positive electrode tin or thenegative electrode tin (705, 808) of the battery and the negativeelectrode cap or the positive electrode cap (704, 809), respectively.The outer tin is preferably made of stainless steel. Other materialsthat can preferably be used for the outer tin include aluminum alloys,titanium-clad stainless materials, copper-clad stainless materials andnickel-plated steel plates.

Since the positive electrode tin (705) in FIG. 7 and the negativeelectrode tin (808) in FIG. 8 operate both as battery housing (case) andas terminal, the use of stainless steel is preferable for the tin.However, when the positive electrode tin or the negative electrode tinis not adapted to operate as terminal, a metal material such as zinc, aplastic material such as polypropylene or a composite material of metalor glass fiber and plastic may be used for the battery housing in placeof stainless steel.

(Safety Valve)

A safety valve is provided in a lithium secondary battery according tothe present invention as a safety measure when the internal pressure ofthe battery rises abnormally. Materials that can be used for the safetyvalve include rubber, a spring, a metal ball and a rupture foil.

(Link Material 903)

Also, the inventors of the present invention believed that theelectrochemical reaction in the electrode layer at the time ofcharging/discharging operations becomes more uniform when the contactresistance between the metal particles and the carbon particles in theelectrode layer is improved and tried to make the electrochemicalreaction more uniform in charging/discharging operations by adding amaterial that functions as “link” and uniformizes the connection ofmetal powder and carbon particles such as graphite particles that aretotally different from each other. Effective link materials that can beused for the purpose of the present invention include coal tar pitch,carbonaceous materials of coal tar pitch and nonionic fluorine typesurfactants.

Coal tar pitch has affinity for both metal particles having ahydrophilic surface and carbon particles having a hydrophobic surfaceand hence electron conduction between metal particles and carbonparticles can be improved by adding coal tar pitch to and mixing it withmetal particles and carbon particles. Preferably, coal tar pitch isadded at amount of 0.1 to 3 wt %. Since the mechanical strength of coaltar pitch per se is poor, the mechanical strength of the electrode layeris reduced when coal tar pitch is added to a large ratio.

Electron conduction between metal particles and carbon particles canalso be improved by adding coal tar pitch to and mixing it with metalparticles and carbon particles, subsequently heat-treating the mixturein a temperature range from 400° C. to 700° C. under an inert gasatmosphere to carbonize the coal tar pitch and produce amorphous carbonand binding the metal particles and the carbon particles by means of theamorphous carbon obtained by the carbonization.

The use of metal particles that are turned amorphous to a large extentis preferable for a lithium secondary battery according to the presentinvention because such metal particles can improve the cycle life of thebattery. A heat-treatment temperature of amorphous metal particles thatexceeds 700° C. is not preferable because crystallization of the metalparticles is accelerated and metal oxide is liable to be formed due tothe adsorbed oxygen and the adsorbed moisture.

The metal particles may be coated with coal tar pitch in advance beforecarbonizing the coal tar pitch.

Preferably, coal tar pitch to be carbonized is added at an amount of 1to 10 wt. % as the link material 903 for binding the metal particles 901selected from silicon, tin or an alloy thereof and carbon particles 902.If coal tar pitch is added at an amount more than the above-definedrange, amorphous carbon is produced to a large extent at the time ofcarbonization. Then, when a negative electrode is formed by the materialobtained by binding the metal particles and carbon particles by means ofthe amorphous carbon that is obtained by carbonizing coal tar pitch anda lithium secondary battery is formed by using such a negativeelectrode, the irreversible capacity of lithium rises to reduce theCoulombic efficiency in the initial charging/discharging cycles.

Preferably, coal tar pitch to be used as the link material shows a lowsoftening point and a high carbonization yield. The softening point ofcoal tar pitch is preferably within a range from 110 to 500° C., morepreferably within a range from 150 to 350° C. For the purpose of thepresent invention, fixed carbon in coal tar pitch is preferably 59 to 90wt %, more preferably 65 to 90 wt %. The weight reduction of coal tarpitch at 1,000° C. is preferably 55% or less. For the purpose of thepresent invention, coal tar pitch preferably contains mesophasespherules at an amount of 2 to 80% at the time of heating the coal tarpitch.

The above-described nonionic fluorine type surfactant can improve thecontact between the metal particles and the carbon particles, the bondsbetween the metal particles and the binder and the bonds between thecarbon particles and the binder when it is added at the time of formingan electrode layer from the metal particles, the carbon particles andthe binder because such a nonionic fluorine type surfactant has C—Fbonds and ether bonds or ester bonds so that it can improve thewettability of both substances having a hydrophilic surface andsubstances having a hydrophobic surface. For the purpose of the presentinvention, the nonionic fluorine type surfactant is added to form anelectrode layer at an amount of 0.01 to 0.5 wt %. Since such asurfactant does not contribute to the charging/discharging reaction ofthe battery, it can degrade the performance of the battery if it isadded to a large extent.

FIG. 9A is a schematic image of metal particles 901 selected fromsilicon, tin or an alloy thereof and a carbon particle 902 connected toeach other by a link material 903 in an electrode structure according tothe present invention. Metal particles 901 and carbon particles 902 canbe bound to each other with ease to facilitate electron conduction byusing coal tar pitch or a fluorine type surfactant as the link material903 that can easily be bonded to the surfaces of metal particles 901having a hydrophilic surface and those of carbon particles 902 having ahydrophobic surface. Then, when it is used for the negative electrodematerial of a lithium secondary battery, it can raise thecharging/discharging efficiency (Coulombic efficiency) and prolong thecharging/discharging service life to effectively improve the performanceof the metal material for occluding lithium to a large extent andrealize a high capacity battery.

FIG. 9B is a schematic image of metal particles 901 and a carbonparticles 902 that are integrally compounded by the link material 903.After mixing metal particles and carbon particles with a material thatis apt to be carbonized such as coal tar pitch, the material that is aptto be carbonized is actually carbonized in inert gas such as nitrogengas or argon gas to obtain metal particles and carbon particles that areintegrally compounded by the carbonaceous material. Then, electronconduction between the metal particles and the carbon particles isfacilitated by the carbonaceous material. Then, when it is used for thenegative electrode material of a lithium secondary battery, it can raisethe charging/discharging efficiency (Coulombic efficiency) and prolongthe charging/discharging service lifetime to effectively improve theperformance of the metal material for occluding lithium to a largeextent and realize a high capacity battery.

Now, the present invention will be described in details with referringto the following Examples.

[Preparation of Electrode Material]

Firstly, a negative electrode material was prepared.

EXAMPLE 1

(1) Preparation of Main Active Material of Negative Electrode

Silicon, tin and copper were mixed at an atomic ratio of 82.9:16.6:0.5(weight ratio of 65:30:5) and molten under an argon gas atmosphere toproduce a melt. Subsequently, powder of an Si—Sn—Cu alloy was obtainedby means of “Water atomization process” of injecting the melt by highlypressurized water. Then, the obtained powder of the Si—Sn—Cu alloy wascrushed in isopropyl alcohol by means of a media mill using a zirconiaball to produce fine powder of the Si—Sn—Cu alloy showing an averageparticle size of 0.3 μm. Then, graphite powder was added to the obtainedfine powder of the Si—Sn—Cu alloy at an amount of 15 wt % and themixture was crushed in an Attriter mill under an argon gas atmosphere bymeans of a stainless steel ball for 10 hours to produce fine powder ofthe Si—Sn—Cu alloy that is compounded with carbon. The obtained Si—Sn—Cualloy-carbon compounded powder was analyzed by means of an X-raydiffractometer to find that it was alloy powder of fine crystals of 30nm that had been turned amorphous.

(2) Preparation of Negative Electrode

The Si—Sn—Cu alloy-carbon compounded powder obtained in the above (1) asthe main active material, artificial graphite powder of pseudo-sphericalparticles with an average particle size of 27 μm, graphite powder withan average particle size of 5 μm as an auxiliary conductive material,and a solution of N-methyl-2-pyrrolidone as a polyamide precursor(polyamic acid) were mixed such that the weight ratio of the compoundedpowder, the artificial graphite powder, the graphite powder and thesolid part of the solution was 74:10:5:11, and N-methyl-2-pyrrolidonewas added as a solvent. The mixture was then kneaded to prepare slurryand the obtained slurry was applied to a copper foil having a ten pointaverage height Rz=0.6 μm and a thickness of 15 μm by means of a coater.The applied slurry was then heat-treated at 150° C. for 30 minutes andsubsequently at 220° C. for 1 hour and then dried at 200° C. underreduced pressure to prepare an electrode structure for negativeelectrode having a negative electrode layer with an average thickness of20 μm and a density of 1.3 g/cm³.

The obtained electrode structure was cut to a predetermined size and anickel ribbon lead was connected to the electrode by spot welding toproduce a negative electrode.

(3) Preparation of Positive Electrode

Lithium-cobalt Oxide LiCoO₂ was mixed with 5 wt % of graphite powder and5 wt % of polyvinylidene fluoride powder and subsequentlyN-methyl-2-pyrrolidone was added to prepare slurry.

The obtained slurry of the positive electrode material was applied to acurrent collector of aluminum foil of a thickness of 20 μm and dried.Then, the current collector carrying the slurry was pressed by a rollpress to make the positive electrode active material layer having athickness of 90 μm and a density of 3.3 g/cm³ at one side of the foil.Then, an aluminum lead was connected to it by means of an ultrasonicwelding machine and dried at 150° C. under reduced pressure to produce apositive electrode.

(4) Preparation Process of Electrolyte Solution

Moisture was thoroughly removed from ethylene carbonate and diethylcarbonate, and the two organic substances were mixed to a volume ratioof 3:7 to prepare a solvent.

1M (mol/liter) of lithium hexafluorophosphate (LiPF₆) was dissolved intothe above solvent to prepare the electrolyte solution of the battery.

(5) Separator

A finely porous film of polyethylene with a thickness of 16 μm was usedas separator.

(6) Assemblage of Battery

The battery was assembled entirely under a dry atmosphere where moisturewas controlled to show a dew point of −50° C. or less.

The separator was sandwiched between the negative electrode and thepositive electrode prepared in a manner as described above and thenegative electrode/separator/positive electrode was put into apocket-shaped battery jar prepared by using aluminum laminate filmhaving a polyethylene/aluminum foil/nylon structure. Then, theelectrolyte solution was poured into the battery jar and the electrodeleads were taken out before the battery jar was heat-sealed to prepare abattery to be used for evaluating the control of the positive electrodecapacity. The nylon film and the polyethylene film of the aluminumlaminate film were made to face the outside and the inside,respectively.

EXAMPLE 2

A battery to be evaluated was prepared in the same manner as in Example1 except that a copper foil having a ten point average height of Rz=2.1μm and a thickness of 15 μm was used for the current collector of thenegative electrode, and the prepared battery was evaluated.

EXAMPLE 3

The process as described below was used in place of the process ofExample 1 to prepare a battery to be evaluated in an evaluation test.

(1) Preparation of Negative Electrode

A solution obtained by dispersing poly(2-ethylthiophene) at an amount of10 wt % in a mixed solvent of ethylene carbonate and diethyl carbonateat a volume ratio of 3:7 and adding the obtained mixture to anN-methyl-2-pyrrolidone solution of a polyamide precursor (polyamic acid)such that poly(2-ethylthiophene) was 10 wt % of the solid part of thepolyamide precursor (polyamic acid).

Then, the Si—Sn—Cu alloy-carbon compounded powder as the main activematerial, artificial graphite powder of pseudo-spherical particles withan average particle size of 22 μm, graphite powder with an averageparticle size of 5 μm as an auxiliary conductive material, and asolution of N-methyl-2-pyrrolidone as a polyamide precursor (polyamicacid) were mixed such that the weight ratio of the Si—Sn—Cu alloy-carboncompounded powder, the artificial graphite powder of pseudo-sphericalparticles with an average particle size of 22 μm, the graphite powderwith an average particle size of 5 μm and the solid part of the solutionof a polyamide precursor (polyamic acid) was 74:10:5:11. Then,N-methyl-2-pyrrolidone was added as a solvent and the mixture waskneaded to prepare slurry. The obtained slurry was applied to a copperfoil having a ten points average height Rz=0.6 μm and a thickness of 15μm by means of a coater. The applied slurry was then heat-treated at150° C. for 30 minutes and subsequently at 220° C. for 1 hour and thendried at 200° C. under reduced pressure to prepare an electrodestructure for negative electrode having a negative electrode layer withan average thickness of 20 μm and a density of 1.3 g/cm³.

The obtained electrode structure was cut to a predetermined size, and anickel ribbon lead was connected to the electrode by spot welding toproduce a negative electrode.

EXAMPLE 4

The process as described below was used in place of the process ofExample 1 to prepare a battery to be evaluated in an evaluation test.

(1) Preparation of Negative Electrode

A buffer layer was formed on a copper foil by way of the followingoperation and subsequently a main active material layer was formedthereon.

Firstly, graphite powder with an average particle size of 5 μm and asolution of a polyamide precursor (polyamic acid) prepared similarly asin Example 3, to which poly(2-ethylthiophene) was added, were mixed suchthat the ratio of the graphite powder and the solid part of the solutionwas 93:7 by weight and then N-methyl-2-pyrrolidone was added as solvent.The mixture was kneaded to prepare slurry and the obtained slurry wasapplied to a copper foil having a ten points average height of Rz=0.6 μmand a thickness of 15 μm by means of a coater. The applied slurry wasthen dried at 150° C. to produce a buffer layer with a thickness of 5 μmand a density of 1.2 g/cm². Poly(2-ethylthiophene) was added to thepolyamide precursor (polyamic acid) functioning as a binder in order toprovide the binder with electron conductivity. This is because theparticles of graphite powder were so small that it was not possible toreduce the amount of the binding material to be used and the particleshave a low electron conductivity, and hence the charging-dischargingefficiency fell in a charging/discharging test at a high electriccurrent density if compared with a charging/discharging test with a lowelectric current density because of the.

Then, Si—Sn—Cu alloy-carbon compounded powder as the main activematerial, artificial graphite powder of pseudo-spherical particles withan average particle size of 22 μm, graphite powder with an averageparticle size of 5 μm as an auxiliary conductive material, and asolution of N-methyl-2-pyrrolidone as a polyamide precursor (polyamicacid) were mixed such that the weight ratio of the compounded powder,the artificial graphite powder, the graphite powder and the solid partof the solution was 74:10:5:11, and N-methyl-2-pyrrolidone was added asa solvent. The mixture was then kneaded to prepare slurry and theobtained slurry was applied to a copper foil on which the buffer layerhad been formed, by means of a coater. The applied slurry was thenheat-treated at 150° C. for 30 minutes and subsequently at 220° C. for 1hour and then dried at 200° C. under a reduced pressure to prepare anelectrode structure for negative electrode having a negative electrodelayer with a thickness of 20 μm and a density of 1.3 g/cm³.

The obtained electrode structure was cut to a predetermined size and anickel ribbon lead was connected to the electrode by spot welding toproduce a negative electrode.

EXAMPLE 5

The process as described below was used in place of the process ofExample 1 to prepare a battery to be evaluated in an evaluation test,

(1) Preparation of Negative Electrode

A coat layer was formed by way of the following operation on anelectrode structure prepared by the process of Example 1.

Firstly, graphite powder with an average particle size of 5 μm and asolution of a polyamide precursor (polyamic acid) prepared similarly asin Example 3, to which poly(2-ethylthiophene) was added, were mixed suchthat the ratio of the graphite powder and the solid part of the solutionwas 93:7 by weight, and then N-methyl-2-pyrrolidone was added assolvent. The mixture was kneaded to prepare slurry.

The obtained slurry was applied to an electrode structure prepared bythe process of Example 2 by means of a coater. The applied slurry wasthen heat-treated at 150° C. for 30 minutes and subsequently at 220° C.for 1 hour and then dried at 200° C. under a reduced pressure to form acoat layer according to the present invention with a thickness of 5 μmand a density of 1.0 g/cm³.

The obtained electrode structure was cut to a predetermined size and anickel ribbon lead was connected to the electrode by spot welding toproduce a negative electrode.

EXAMPLE 6

A buffer layer of a negative electrode was formed by carrying out thefollowing operation, in place of the operation (1) in Example 4.Firstly, graphite powder with an average particle size of 5 μm, tungsticoxide WO₃ powder with an average particle size of 5 μm and a solution ofa polyamide precursor (polyamic acid) prepared similarly as in Example3, to which poly(2-ethylthiophene) was added, were mixed such that theweight ratio of the graphite powder, the WO₃ powder and the solid partof the solution was 50:43:7, and then N-methyl-2-pyrrolidone was addedas a solvent. The mixture was kneaded to prepare slurry and the obtainedslurry was applied to a copper foil having a ten point average height ofRz=0.6 μm and a thickness of 15 μm by means of a coater. The appliedslurry was then dried at 150° C. to produce a buffer layer with athickness of 5 μm and a density of 1.3 g/cm³.

Subsequently, the other operations for preparing the battery werecarried out similarly to the process of Example 4 to prepare a batteryto be evaluated in an evaluation test.

EXAMPLE 7

A main active material layer was formed on a buffer layer similarly asin Example 6 and heat-treated at 150° C. for 30 minutes, but, beforefurther heat-treating the buffer layer at 220° C. for 1 hour, a surfacecoat layer was formed by way of the following operation. Graphite powderwith an average particle size of 5 μm functioning as auxiliaryconductive material, lithium-titanium oxide Li_(4/3)Ti_(5/3)O₂ powderwith an average particle size of 4 μm prepared by using titanium oxideand lithium carbonate as raw materials and a solution of a polyamideprecursor (polyamic acid) prepared similarly as in Example 3, to whichpoly(2-ethylthiophene) was added, were mixed such that the weight ratioof the graphite powder, the Li_(4/3)Ti_(5/3)O₂ powder and the solid partof the solution was 60:33:7, and then N-methyl-2-pyrrolidone was addedas a solvent. The mixture was kneaded to prepare slurry and the obtainedslurry was applied to an electrode structure prepared by the process ofExample 2 by means of a coater. The applied slurry was then heat-treatedat 150° C. for 30 minutes and subsequently at 220° C. for 1 hour andthen dried at 200° C. under a reduced pressure to form a coat layeraccording to the present invention with a thickness of 5 μm and adensity of 1.1 g/cm³. Thus, a three-layered electrode structure for anegative electrode was prepared.

Subsequently, the other operations for the battery were carried outsimilar to the process of Example 6 to prepare a battery to be evaluatedin an evaluation test.

EXAMPLE 8

Electrolyte solution was prepared by adding 2 parts of styrene and 2parts of vinylene carbonate to 100 parts of ethylene carbonate/diethylcarbonate solution of 1M(mol/liter) lithium hexafluorophosphate (LiPF₆)by weight, and the same process of Example 1 was used to prepare abattery to be evaluated in an evaluation test.

EXAMPLE 9

The same process as of Example 2 was used to prepare a battery to beevaluated in an evaluation test except that the binder obtained byadding a conductive polymer to the binder of Example 3 and theelectrolyte solution obtained by adding styrene and vinylene carbonateto the electrolyte solution of Example 8 were used.

EXAMPLE 10

(1) Preparation of Main Active Material of Negative Electrode

Si—Sn—Cu alloy-carbon compounded powder (melting point: 150° C.,carbonized at 700° C.) which was the same as the powder obtained in theoperation (1) of preparation of the main active material of Example 1was coated with coal tar pitch and then held under a nitrogen atmosphereat 600° C. for 1 hour. Subsequently, it was heat-treated at 700° C. for1 hour to carbonize the coal tar pitch and prepare carbon-coated (5 wt%) Si—Sn—Cu alloy-carbon compounded powder,

(2) Preparation of Negative Electrode

The process (2) in Example 1 was used to prepare a negative electrode byusing the carbon-coated Si—Sn—Cu alloy-carbon compounded powder, whichwas the main active material, obtained in the above process (1), inplace of the Si—Sn—Cu alloy-carbon compounded powder of (2) in Example1.

Subsequently, the process of Example 1 was used to prepare a battery tobe evaluated in an evaluation test.

EXAMPLE 11

The process of Example 3 was used to prepare a battery to be evaluated,except that the copper foil of the current collector was replaced by acopper coil having a ten point average height of Rz=2.1 μm and athickness of 15 μm, and the electrolyte solution obtained by addingstyrene and vinylene carbonate to the electrolyte solution of Example 8was used.

Reference Example 1

An electrode structure to be used for a negative electrode was preparedby the same process as in Example 1 except that graphite powder of flatparticles (substantially disk-shaped graphite particles with a diameterof about 5 μm and a thickness of about 1 μm) was used in place of theartificial graphite powder of pseudo-spherical particles with an averageparticle size of 22 μm.

Otherwise, the process of Example 1 was used to prepare a battery to beevaluated.

Comparative Example 1

An electrode structure to be used for a negative electrode was preparedusing a process as described below.

As main active material, artificial graphite powder of pseudo-sphericalparticles with an average particle size of 22 μm, and a solution ofN-methyl-2-pyrrolidone as a polyamide precursor (polyamic acid) weremixed such that the weight ratio of the powder and the solid part of thesolution was 89:11. Then, N-methyl-2-pyrrolidone was added as a solvent,and the mixture was kneaded to prepare a slurry. The obtained slurry wasapplied to a copper foil having a ten point average height of Rz=0.6 μmand a thickness of 15 μm by means of a coater. The applied slurry wasthen heat-treated at 150° C. for 30 minutes and subsequently at 220° C.for 1 hour and then dried at 200° C. under a reduced pressure to preparean electrode structure for a negative electrode having a negativeelectrode layer with a thickness of 20 μm and a density of 1.4 g/cm³.

The obtained electrode structure was cut to a predetermined size and anickel ribbon lead was connected to the electrode by spot welding toproduce a negative electrode.

The same process as in Example 1 was used to prepare a battery to beevaluated in an evaluation test except for the preparation of thenegative electrode.

[Evaluation of Batteries]

A charging/discharging test was conducted on each battery by repeating acycle of constant current-constant voltage charging: charging to 4.2V bya constant current at a rate of 1 C and, when the battery voltagereached to 4.2V, shifting to constant voltage charging of 4.2V, anddischarging by a constant current at rate of 1C down to a batteryvoltage of 3.0V, referring to the capacity of the positive electrode,for 100 times. A pause of 30 minutes was provided when switching fromcharging to discharging and when switching from discharging to charging.The first discharged capacity, the ratio of the first dischargedcapacity to the first charged capacity (charging-dischargingefficiency=Coulombic efficiency), the 100th discharged capacity and theratio of the 100th discharged capacity to the 100th charged capacitywere observed to evaluate the battery by comparing the above values withthe corresponding values of Reference Example 1, normalizing each of thevalues of Reference Example 1 to 1.00. Table 1 summarily shows theobtained results.

The capacity to be charged of the battery to be evaluated of ComparativeExample 1 was limited so as not to exceed the capacity of the negativeelectrode that was computationally determined in advance because thestorage capacity of the negative electrode of Comparative Example 1 waslow.

TABLE 1 Normalized values based on values of Reference Example 1 beingnormalized to 1.00 1st 100th 1st charging- 100th charging- dischargingdischarging charged discharging Example quantity efficiency quantityefficiency Example 1 1.00 1.25 1.52 1.03 Example 2 1.00 1.25 2.54 1.03Example 3 1.00 1.23 2.51 1.04 Example 4 1.00 1.21 1.87 1.03 Example 51.00 1.17 1.75 1.02 Example 6 1.00 1.19 1.95 1.03 Example 7 0.98 1.152.06 1.02 Example 8 1.00 1.07 2.48 1.03 Example 9 1.00 1.20 3.05 1.04Example 10 0.95 1.15 1.98 1.03 Example 11 1.00 1.25 3.24 1.04 Ref. Ex. 11.00 1.00 1.00 1.00 Com. Ex. 1 0.24 1.26 1.02 1.04

The discharged capacity per unit weight of the electrode layer (exceptfor the weight of the current collector) maximally showed a value of1,300 mAh/g or more in Example 1.

From Table 1, it is clear that all the batteries of Examples 1 through11 operated better than the battery of Reference Example 1 in terms ofthe first discharged capacity, the ratio of the first dischargedcapacity to the first charged capacity, the 100th discharged capacity,and the ratio of the 100th discharged capacity to the 100th chargedcapacity.

By comparing with the battery of Comparative Example 1 where graphitewas used for the negative electrode, it is also clear that the batteriesof Examples 1 through 11 operated better than the battery of ComparativeExample 1 in terms of the first discharged capacity and the 100thdischarged capacity, although the battery of Comparative Example 1exbatteryed the batteries of Examples 1 to 11 in terms of the ratio ofthe first discharged capacity to the first charged capacity.

Additionally, batteries to be evaluated in an evaluation test wereprepared by using different materials for the positive electrode inorder to raise the battery voltage at the time of discharging andimprove the safety.

EXAMPLE 12

A battery to be evaluated was prepared similarly as in Example 11 exceptthat a positive electrode prepared by a process as described below wasused.

(3) Preparation of Positive Electrode

35 wt % of spinel type lithium manganese oxide LiMn_(1.5)Ni_(0.5)O₄ and55 wt % of zirconium-added lithium cobalt oxide LiCo_(0.96)Zr_(0.04)O₂were mixed with 5 wt % of graphite powder and 5 wt % of powderypolyvinylidene fluoride, and N-methyl-2-pyrrolidone was added to themixture to prepare a slurry.

The obtained positive electrode material slurry was applied to a currentcollector of aluminum foil with a thickness of 20 μm and dried.Subsequently, the slurry on the current collector was pressed by a rollpress to make the positive electrode active material layer having athickness of 90 μm and a density of 3.3 g/cm³ on one side of the foil.Then, an aluminum lead was connected to it by means of an ultrasonicwelding machine and dried at 150° C. under reduced pressure to produce apositive electrode.

EXAMPLE 13

A battery to be evaluated was prepared similarly as in Example 11 exceptthat a positive electrode prepared by a process as described below wasused.

(1) Preparation of Positive Electrode

45 wt % of LiCo_(0.33)Ni_(0.34)Mn_(0.33)O₂ and 45 wt % ofzirconium-added lithium cobalt oxide LiCo_(0.96)Zr_(0.04)O₂ were mixedwith 5 wt % of graphite powder and 5 wt % of powdery polyvinylidenefluoride, and then N-methyl-2-pyrrolidone was added to the mixture toprepare a slurry.

The obtained positive electrode material slurry was applied to a currentcollector of aluminum foil with a thickness of 20 μm and dried.Subsequently, the slurry on the current collector was pressed by a rollpress to make the positive electrode active material layer having athickness of 90 μm and a density of 3.3 g/cm³ on one side of the foil.Then, an aluminum lead was connected to it by means of an ultrasonicwelding machine and dried at 150° C. under reduced pressure to produce apositive electrode.

EXAMPLE 14

A battery to be evaluated was prepared similarly as in Example 11 exceptthat a positive electrode prepared by a process as described below wasused.

(1) Preparation of Positive Electrode

40 wt % of spinel type lithium manganese oxide LiMn_(1.5)Ni_(0.5)O₄ and50 wt % of LiNi_(0.34)Co_(0.33)Mn_(0.33)O₂ were mixed with 5 wt % ofgraphite powder and 5 wt % of powdery polyvinylidene fluoride, and thenN-methyl-2-pyrrolidone was added to the mixture to prepare a slurry.

The obtained positive electrode material slurry was applied to a currentcollector of aluminum foil with a thickness of 20 μm and dried.Subsequently, the slurry on the current collector was pressed by a rollpress to make the positive electrode active material layer having athickness of 90 μm and a density of 3.3 g/cm³ at a side of the foil.Then, an aluminum lead was connected to it by means of an ultrasonicwelding machine and dried at 150° C. under a reduced pressure to producea positive electrode.

EXAMPLE 15

A battery to be evaluated was prepared similarly as in Example 11 exceptthat a positive electrode prepared in a process as described below wasused.

(1) Preparation of Positive Electrode

30 wt % of spinel type lithium manganese oxide LiMn_(1.5)Ni_(0.5)O₄, 10wt % of LiMn₂O₄ and 50 wt % of LiNi_(0.34)Co_(0.33)Mn_(0.33)O₂ weremixed with 5 wt % of graphite powder and 5 wt % of powderypolyvinylidene fluoride, and then N-methyl-2-pyrrolidone was added tothe mixture to prepare a slurry.

The obtained positive electrode material slurry was applied to a currentcollector of aluminum foil with a thickness of 20 μm and dried.Subsequently, the slurry on the current collector was pressed by a rollpress to make the positive electrode active material layer having athickness of 90 μm and a density of 3.3 g/cm³ on one side of the foil.Then, an aluminum lead was connected to it by means of an ultrasonicwelding machine and dried at 150° C. under a reduced pressure to producea positive electrode.

[Evaluation of Batteries]

The discharging capacity and the average discharging voltage of each ofthe batteries obtained in Examples 12 through 15 were evaluated bycomparing them with the batteries obtained in Example 11 and ComparativeExample 1 by way of the process as described below.

A charging/discharging test was conducted on each battery by repeating acycle of constant current-constant voltage charging: charging to 4.6V bya constant current at a rate of 1C and, when the battery voltage reachedto 4.6V, shifting to constant voltage charging of 4.6V, and dischargingby a constant current at rate of 0.2C down to a battery voltage of2.75V, referring to the capacity of the positive electrode of Example11. A pause of 30 minutes was provided when switching from charging todischarging and when switching from discharging to charging. The firthdischarged capacity was observed and the 5th average discharge voltagewas determined for each battery to evaluate the battery by comparing theabove values with the corresponding values of Reference Example 11,normalizing each of the values of Example 11 to 1.00. Table 2 summarilyshows the obtained results.

TABLE 2 Normalized values based on values of Reference Example 11 beingnormalized to 1.00 5th discharged 5th average Example quantity dischargevoltage Example 12 1.07 1.15 Example 13 1.25 1.00 Example 14 1.11 1.13Example 15 1.12 1.09 Example 11 1.00 1.00 Comp. Example 1 0.24 1.12

From the results summarized in Table 2, it was found that the dischargevoltage is raised and the discharged capacity is increased byappropriately adding lithium-manganese-nickel oxide,lithium-cobalt-nickel-manganese oxide and/or lithium-cobalt oxide.

[Safety Evaluation Test]

The safety of each of the batteries of Examples 12 through 15 wasevaluated by comparing it with the battery of Example 11 in thefollowing manner. The battery was charged by a constant current at arate of 1C up to 5.0V, referring to the capacity of the positiveelectrode of the battery of Example 11. Then, each of the batteries wasevaluated by means of accelerating rate calorimetry (ARC). As a result,it was found that the battery of Example 11 had the lowest heat-emissionstarting temperature, and that the heat-emitting rate at and near 100°C. was ranked by the order of Example 11, Example 13, Example 12,Example 14 and Example 15. Thus, it was found that the safety of batteryis improved by the positive electrode containing one or more manganesecompounds.

From Table 2 and the results of the safety evaluation test, it was foundthat a lithium secondary battery having a negative electrode formed byusing a negative electrode material containing silicon, tin or an alloythereof is made safe and to show a high energy density when combinedwith an appropriate positive electrode material.

EXAMPLE 16

A battery to be evaluated was prepared similarly as in Example 11 exceptthat a positive electrode prepared by a process as described below wasused.

(1) Preparation of Positive Electrode

LiCo_(0.33)Ni_(0.34)Mn_(0.33)O₂ and zirconium-added lithium cobalt oxideLiCo_(0.96)Zr_(0.04)O₂ were mixed such that a weight ratio thereof was50:50, and 10 parts of coal tar pitch (with a melting point of 350° C.to be carbonized at 700° C.) was added to 100 parts of the mixture.Then, the mixture was treated under an argon gas atmosphere at 400° C.for 1 hour and subsequently at 700° C. for 1 hour to obtainLiCo_(0.33)Ni_(0.34)Mn_(0.33)O₂ and LiCo_(0.96)Zr_(0.04)O₂ that werecoated with carbonaceous material of coal tar pitch. The obtainedproduct contained carbonaceous material of coal tar pitch by 2 wt %.

Thereafter, the obtained product of LiCo_(0.33)Ni_(0.34)Mn_(0.33)O₂ andLiCo_(0.96)Zr_(0.04)O₂ that were coated with carbonaceous material ofcoal tar pitch was mixed with 5 wt % of graphite powder and 5 wt % ofpowdery polyvinylidene fluoride, and then N-methyl-2-pyrrolidone wasadded to the mixture to prepare a slurry.

The obtained positive electrode material slurry was applied to a currentcollector of aluminum foil with a thickness of 20 μm and dried.Subsequently, the slurry on the current collector was pressed by a rollpress to make the positive electrode active material layer having athickness of 90 μm and a density of 3.3 g/cm³ on one side of the foil.Then, an aluminum lead was connected to it by means of an ultrasonicwelding machine and dried at 150° C. under a reduced pressure to producea positive electrode.

The high rate discharging performance of the battery obtained in Example16 was evaluated by comparing it with the battery of Example 11 in thefollowing manner.

A charging/discharging test was conducted on each battery by repeating acycle of constant current-constant voltage charging: charging to 4.6V bya constant current at a rate of 1 C and, when the battery voltagereached to 4.6V, shifting to constant voltage charging of 4.6V, anddischarging by a constant current at rate of 2.0 C down to a batteryvoltage of 2.75V, referring to the capacity of the positive electrode ofExample 11. The firth discharged capacity was observed to find that thedischarged capacity of the battery of Example 16 was 1.2 times that ofthe battery of Example 11. From this point, it was found that theperformance of a battery is improved in efficient discharging at anelevated current density by coating the particles of the positiveelectrode active material with the carbonized coal tar pitch.

Reference Example 2

(1) Preparation of Silicon Alloy to be Used for Main Material ofNegative Electrode Layer

As metal powder to be used for forming a negative electrode, fine powderof silicon was obtained by crushing metal silicon powder in isopropylalcohol in a medial mill using zirconia beads such that the powder hadan average particle size of 0.2 μm.

The fine powder of silicon was mixed with tin powder, copper powder,boron powder and graphite powder such that a weight ratio thereof was58.5:27.0:4.5:1.0:9.0, and the mixture was crushed in an Attriter millunder an argon gas atmosphere by means of a stainless steel ball for 24hours to produce fine powder of the Si—Sn—Cu—B alloy that was compoundedwith carbon as the electrode material.

(2) Preparation of Negative Electrode

The Si—Sn—Cu alloy-carbon compounded powder obtained in the above (1) asthe main active material, artificial graphite powder of pseudo-sphericalparticles with an average particle size of 27 μm, graphite powder withan average particle size of 5 μm as an auxiliary conductive material,and a solution of N-methyl-2-pyrrolidone of polyamideimide were mixedsuch that the weight ratio of the compounded powder, the artificialgraphite powder, the graphite powder and the solid part of the solutionwas 74:10:5:11, and N-methyl-2-pyrrolidone was added as a solvent. Themixture was then kneaded to prepare a slurry and the obtained slurry wasapplied to a copper foil having a ten point average height of Rz=0.6 μmand a thickness of 15 μm by means of a coater. The applied slurry wasthen heat-treated at 150° C. for 30 minutes and then dried at 200° C.under a reduced pressure to prepare an electrode structure for negativeelectrode having a negative electrode layer with an average thickness of20 μm and a density of 1.3 g/cm³.

The obtained electrode structure was cut to a predetermined size and anickel ribbon lead was connected to the electrode by spot welding toproduce a negative electrode,

(3) Preparation of Positive Electrode

Lithium-cobalt oxide LiCoO₂ was mixed with 5 wt % of graphite powder and5 wt % of polyvinylidene fluoride powder and subsequentlyN-methyl-2-pyrrolidone was added to the mixture to prepare a slurry.

The obtained slurry of the positive electrode material was applied to acurrent collector of aluminum foil of a thickness of 20 μm and dried.Then, the slurry on the current collector was pressed by a roll press tomake the positive electrode active material layer having a thickness of90 μm and a density of 3.3 g/cm³ on one side of the foil. Then, analuminum lead was connected to it by means of an ultrasonic weldingmachine and dried at 150° C. under a reduced pressure to produce apositive electrode.

(4) Preparation Process of Electrolyte Solution

Moisture was thoroughly removed from ethylene carbonate and diethylcarbonate, and these solvents were mixed at a volume ratio of 3:7 toprepare a mixed solvent.

Lithium hexafluorophosphate (LiPF₆) was dissolved at 1M (mol/liter) intothe above solvent to prepare the electrolyte solution of the battery.

(5) Separator

A finely porous film of polyethylene with a thickness of 16 μm was usedas a separator.

(6) Assemblage of Battery

The battery was assembled entirely under a dry atmosphere where moisturewas controlled so as to show a dew point of −50° C. or less.

The separator was sandwiched between the negative electrode and thepositive electrode prepared in a manner as described above and thenegative electrode/separator/positive electrode were put into apocket-shaped battery jar prepared by using aluminum laminate filmhaving a polyethylene/aluminum foil/nylon structure. Then, theelectrolyte solution was poured into the battery jar and the electrodeleads were taken out before the battery jar was heat-sealed to prepare abattery to be used for evaluating the control of the positive electrodecapacity. The nylon film and the polyethylene film of the aluminumlaminate film were arranged to face the outside and the inside,respectively.

EXAMPLE 17

(1) Preparation of Main Material of Negative Electrode Layer

Si—Sn—Cu alloy-carbon compounded powder obtained in the above (1) ofReference Example 1, artificial graphite powder of pseudo-sphericalparticles with an average particle size of 27 μm, graphite powder withan average particle size of 5 μm as an auxiliary conductive material,and coal tar pitch MCP-350 (tradename, available from JFE ChemicalCorporation, softening temperature: 350° C., solid carbon: 88%, weightloss at 1,000° C.: 16%) were mixed such that a weight ratio thereof was74:10:5:1, and isopropyl alcohol was added to the mixture. Then, themixture was mixed further in a planetary ball mill, using an agate-madevessel and a ball, and the obtained mixture was dried at 80° C. toobtain alloy-graphite-coal tar pitch compounded powder.

(2) Preparation of Negative Electrode

The alloy-graphite-coal tar pitch compounded powder obtained in theabove (1) and a solution of N-methyl-2-pyrrolidone of polyamideimidewere mixed such that the weight ratio of the powder and the solid partof the solution was 89:11, and then N-methyl-2-pyrrolidone was added asa solvent. The mixture was then kneaded to prepare slurry and theobtained slurry was applied to a copper foil having a ten point averageheight of Rz=0.6 μm and a thickness of 15 μm by means of a coater. Theapplied slurry was then heat-treated at 150° C. for 30 minutes and thendried at 200° C. under a reduced pressure to prepare an electrodestructure for a negative electrode having a negative electrode layerwith an average thickness of 20 μm and a density of 1.3 g/cm³.

The obtained electrode structure was cut to a predetermined size and anickel ribbon lead was connected to the electrode by spot welding toproduce a negative electrode.

Thereafter, the same process as in Reference Example 2 was used toprepare a battery to be evaluated.

EXAMPLE 18

(1) Preparation of Main Material of Negative Electrode Layer

Si—Sn—Cu alloy-carbon compounded powder obtained in the above (1) ofReference Example 2, artificial graphite powder of pseudo-sphericalparticles with an average particle size of 27 μm, graphite powder withan average particle size of 5 μm as an auxiliary conductive material,and coal tar pitch MCP-350 (tradename, available from JFE ChemicalCorporation, softening temperature: 350° C., solid carbon: 88%, weightloss starting temperature: 420° C., weight loss at 1,000° C.: 16%) weremixed such that a weight ratio thereof was 74:10:5:10, and isopropylalcohol was added to the mixture. Then, the mixture was mixed further ina planetary ball mill, using an agate-made vessel and a ball, and theobtained mixture was dried at 80° C. to obtain alloy-graphite-coal tarpitch compounded powder. The obtained compounded powder was heat-treatedat 550° C. for 1 hour under an N₂ gas flow to carbonize the coal tarpitch into amorphous carbon and obtain alloy-carbonized coal tarpitch-graphite compounded powder.

(2) Preparation of Negative Electrode

The alloy-carbonized coal tar pitch-graphite compounded powder obtainedin the above (1) and a solution of N-methyl-2-pyrrolidone that was apolyamideimide were mixed such that the weight ratio of the powder andthe solid part of the solution was 89:11, and thenN-methyl-2-pyrrolidone was added as a solvent. The mixture was thenkneaded to prepare slurry and the obtained slurry was applied to acopper foil having a ten point average height of Rz=0.6 μm and athickness of 15 μm by means of a coater. The applied slurry was thenheat-treated at 150° C. for 30 minutes and then dried at 200° C. under areduced pressure to prepare an electrode structure for negativeelectrode having a negative electrode layer with an average thickness of20 μm and a density of 1.3 g/cm³.

The obtained electrode structure was cut to a predetermined size and anickel ribbon lead was connected to the electrode by spot welding toproduce a negative electrode.

Thereafter, the same process as in Reference Example 2 was used toprepare a battery to be evaluated.

EXAMPLE 19

(1) Preparation of Main Material of Negative Electrode Layer

Si—Sn—Cu alloy-carbon compounded powder obtained in the above (1) ofReference Example 2, coal tar pitch MCP-350 (tradename, available fromJFE Chemical Corporation, softening temperature: 350° C., solid carbon:88%, weight loss starting temperature: 420° C., weight loss at 1,000°C.: 16%), and surfactant Novec FC-4430 (tradename, available from 3M)which is fluorine type aliphatic polymer ester were mixed such that aweight ratio thereof was 95:5:0.5, and acetone was added to the mixture.Then, the mixture was mixed further in a planetary ball mill, using anagate-made vessel and a ball, and the obtained mixture was dried at 50°C. to obtain coal-tar-pitch-coated alloy-carbon compounded powder. Theobtained alloy powder was heat-treated at 700° C. for 1 hour under anargon gas flow to carbonize the coal tar pitch into amorphous carbon andobtain amorphous-carbon-coated alloy-carbon compounded powder.

(2) Preparation of Negative Electrode

The amorphous-carbon-coated alloy-carbon compounded powder obtained inthe above (1), artificial graphite powder of pseudo-spherical particleswith an average particle size of 27 μm, graphite powder with an averageparticle size of 5 μm as an auxiliary conductive material, andsurfactant Novec FC-4430 (tradename, available from 3M) which isfluorine type aliphatic polymer ester were mixed such that a weightratio thereof was 74:10:5:0.1. Then, a solution ofN-methyl-2-pyrrolidone was added as a solvent, and the mixture was mixedfurther. Subsequently, the solid part of the solution ofN-methyl-2-pyrrolidone of polyamideimide was added to theamorphous-carbon-coated alloy-carbon compounded powder such that theweight ratio of the solid part of the N-methyl-2-pyrrolidone solutionand powder was 11:74. The mixture was then kneaded to prepare slurry andthe obtained slurry was applied to a copper foil having a ten pointaverage height of Rz=0.6 μm and a thickness of 15 μm by means of acoater. The applied slurry was then heat-treated at 150° C. for 30minutes and then dried at 200° C. under a reduced pressure to prepare anelectrode structure for a negative electrode having a negative electrodelayer with an average thickness of 20 μm and a density of 1.3 g/cm³.

The obtained electrode structure was cut to a predetermined size and anickel ribbon lead was connected to the electrode by spot welding toproduce a negative electrode.

Thereafter, the same process as in Reference Example 2 was used toprepare a battery to be evaluated.

EXAMPLE 20

The process (2) of preparing a negative electrode in Reference Example 2was used in a manner as described below.

Si—Sn—Cu alloy-carbon compounded powder obtained in the above (1) ofReference Example 2, artificial graphite powder of pseudo-sphericalparticles with an average particle size of 27 μm graphite powder with anaverage particle size of 5 μm as an auxiliary conductive material, andsurfactant Novec FC-4430 (tradename, available from 3M) which isfluorine type aliphatic polymer ester were mixed such that a weightratio thereof was 74:10:5:0.1. Then, a solution ofN-methyl-2-pyrrolidone was added as a solvent, and the mixture was mixedfurther. Subsequently, the solid part of the solution ofN-methyl-2-pyrrolidone of polyamideimide was added to the mixture suchthat the weight ratio of the solid part of the N-methyl-2-pyrrolidonesolution to the Si—Sn—Cu alloy-carbon compounded powder was 11:74.

The mixture was then kneaded to prepare a slurry and the obtained slurrywas applied to a copper foil having a ten point average height of Rz=0.6μm and a thickness of 15 μm by means of a coater. The applied slurry wasthen heat-treated at 150° C. for 30 minutes and then dried at 200° C.under a reduced pressure to prepare an electrode structure for anegative electrode having a negative electrode layer with an averagethickness of 20 μm and a density of 1.3 g/cm³.

The obtained electrode structure was cut to a predetermined size and anickel ribbon lead was connected to the electrode by spot welding toproduce a negative electrode.

Thereafter, the same process as in Reference Example 2 was used toprepare a battery to be evaluated.

[Evaluation of Batteries]

A charging/discharging test was conducted on each battery by repeating acycle of constant current-constant voltage charging: charging to 4.2V bya constant current at a rate of 1C and, when the battery voltage reachedto 4.2V, shifting to constant voltage charging of 4.2V, and dischargingby a constant current at rate of 1C down to a battery voltage of 2.5V,referring to the capacity of the positive electrode, for 100 times. Apause of 30 minutes was provided when switching from charging todischarging and when switching from discharging to charging. The 100thdischarged capacity relative to the 1st discharged quantity wasevaluated as capacity maintaining ratio for each battery and the ratiowas compared with the corresponding ratio of Reference Example 2,normalizing the value of Reference Example 2 to 1.0. Table 3 summarilyshows the obtained results.

TABLE 3 Capacity maintaining ratio of 100th cycle (%)/capacitymaintaining ratio of Reference Example 2 (%) Reference 1.0 Example 2Example 17 1.3 Example 18 1.4 Example 19 1.4 Example 20 1.2

As described above, the present invention provides a lithium secondarybattery having a large capacity, a high energy density and a long cyclelife.

Thus, a lithium secondary battery can realize a high storage capacityand a high energy density as well as a long cycle life.

This application claims priority from Japanese Patent Application No.2004-358458 filed on Dec. 10, 2004, which is hereby incorporated byreference herein.

1. An electrode structure for a lithium secondary battery, comprising: amain active material layer comprising: a metal powder of a materialselected from the group consisting of silicon, tin and an alloy thereofthat can store and discharge lithium by electrochemical reaction; hardcarbon powder or carbon powder of graphite; and a binder of an organicpolymer; and a current collector, wherein the metal powder and thecarbon powder are compounded by a link material having a function ofcarrying out chemical bonding or electron conduction between the metalpowder and the carbon powder, and wherein the link material is anonionic fluorine surfactant having an ether bond or an ester bond. 2.The electrode structure according to claim 1, wherein the fluorinesurfactant is contained at an amount of 0.01 to 0.5 wt % in the mainactive material layer.
 3. A lithium secondary battery comprising anegative electrode formed by using an electrode structure according toclaim 1, a lithium ion conductor, and a positive electrode, wherein anoxidation reaction of lithium and a reduction reaction of lithium ionsare utilized.
 4. A method of producing the electrode structure accordingto claim 1, comprising: a step of mixing the metal powder and the carbonpowder with a fluorine type surfactant at an amount of 0.1 to 0.5 wt %;and a step of adding the binder to a mixture obtained in the abovemixing step, mixing them, and applying the obtained mixture onto aplate-shaped metal current collector.
 5. The method according to claim4, wherein in the step of adding the binder to the mixture, mixing themand applying the obtained mixture onto the plate-shaped metal currentcollector, a solvent for the binder is added to and mixed with themixture at the time of mixing the binder with the mixture.
 6. A methodof producing a lithium secondary battery, comprising a step ofinterposing a lithium ion conductor between a negative electrode formedby using an electrode structure produced by the method according toclaim 4 and a positive electrode.